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.

README.md

@@ -93,7 +93,7 @@ readable version of these programs, you should next consider some preprocessing
 such as:
 
 ``` <!---sh-->
-sed -e '/^#.*include/d' prog.c | cc -E
+    sed -e '/^#.*include/d' prog.c | cc -E
 ```
 
 This strips away comments and expands the program's macros without having things