fix non-indented markdown code blocks

We have fixed cases where markdown code blocks were not properly intended.

Updated `markdown.md` to discuss how markdown code blocks should be indented
and how blank lines should follow a markdown code block.

We used this shell script to find problematic code blocks:

```sh
find . -name '*.md' ! -name markdown.md -print | xargs pcregrep -M '^``.*\n[^|`\n \t]'
```

Ran `make www` to perform the above.

2018/burton2/README.md

@@ -343,7 +343,7 @@ NB: The keyword file used in this code is derived from the list in [iocccsize.c]
 which is neither complete (`#define`, `#ifndef`, `#undef` are missing
 
 ```
-yes, Virginia knows about `#define` omitted on purpose
+    yes, Virginia knows about `#define` omitted on purpose
 ```
 
 nor correct (many more are added: `I`, `true`, `bool`, `compl`, ...):

2018/burton2/index.html

@@ -644,7 +644,7 @@ the IOJCC trash can.</p>
 </ul>
 <p>NB: The keyword file used in this code is derived from the list in <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2018/iocccsize.c">iocccsize.c</a>,
 which is neither complete (<code>#define</code>, <code>#ifndef</code>, <code>#undef</code> are missing</p>
-<pre><code>yes, Virginia knows about `#define` omitted on purpose</code></pre>
+<pre><code>    yes, Virginia knows about `#define` omitted on purpose</code></pre>
 <p>nor correct (many more are added: <code>I</code>, <code>true</code>, <code>bool</code>, <code>compl</code>, …):</p>
 <pre><code>    (sed -n &#39;1p&#39; c11; sed -n &#39;2,$p&#39; ioccc.kw.freq | sort) | comm -3 - c11</code></pre>
 <p>That is an obfuscated way to say “diff”. But it also more clearly shows the, um, diffs.</p>

2018/ferguson/README.md

@@ -744,7 +744,7 @@ Keep in mind the way I display the generations and the fact that generation 0
 has no parents; it is generation 1 that starts working towards the goal. You'll
 see also **(mutation)** at the end of the line at this point (if you enable
 monkey mode you'll find that it shows something else). The lines that only have
-`` are separators: you won't see this between generation 0 and 1 but you will
+back-quote ("\`") are separators: you won't see this between generation 0 and 1 but you will
 between future generations - as well as the final answer (if the answer is in
 generation 0 you won't see the ``).
 

2018/ferguson/index.html

@@ -895,7 +895,9 @@ bash shell built-in utility: that isn’t output from my entry.</p>
 has no parents; it is generation 1 that starts working towards the goal. You’ll
 see also <strong>(mutation)</strong> at the end of the line at this point (if you enable
 monkey mode you’ll find that it shows something else). The lines that only have
-<code>are separators: you won't see this between generation 0 and 1 but you will between future generations - as well as the final answer (if the answer is in generation 0 you won't see the</code>).</p>
+back-quote (“`”) are separators: you won’t see this between generation 0 and 1 but you will
+between future generations - as well as the final answer (if the answer is in
+generation 0 you won’t see the ``).</p>
 <div id="test">
 <p>One final thing to consider is that <strong>the program doesn’t directly read from a
 file</strong>. This combined with the way the shell works could possibly confuse some