Added line-height rule for .comments to stop line numbering going out of sync

Issue with commented code in code examples in the docs, cause they're italic they seem to be making the `<code>` blocks be 15px instead of 14px. So I have added in a line-height rule to reel it back in.

Sublime Text has gotten rid of a few bits of whitespace too.

My edit can be found on [line 297](https://github.com/dangerdan/silverstripe-docsviewer/commit/19fe346db95545cfae4ffc96153a39e6c8abdf11#L0R293)