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.

2012/tromp/README.md

@@ -373,6 +373,7 @@ and change it to BLC8 with
     $ wc rev.Blc
 	0 1 9 rev.Blc
 ```
+
 and then try it out with:
 
 ``` <!---sh-->