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.

2019/mills/README.md

@@ -412,7 +412,6 @@ performance, at the cost of a much larger parameter space.
 
 **IMPORTANT NOTE**:
 
-```
 Since OMLET uses the system stack for network storage,
 larger networks may cause OMLET to crash (typically with a message like
 Segmentation fault) unless the system stack size is first increased.
@@ -422,7 +421,6 @@ with ulimit -Hs and set it with ulimit -s 65532 (replacing 65532
 with the actual hard limit).  On csh/tcsh shells, you can view
 the hard limit with limit -h stacksize and set it with
 limit stacksize 65532 (replacing 65532 with the actual hard limit).
-```
 
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