Fixed type for dumpLineNumbers via media query

I am finishing up writing an options panel for less2css.org and noticed a typo on the lesscss.org usage docs.

It currently states possible options for `dumpLineNumbers` as "comments", "mediaQuery", and "all"; however, the source accepts only all-lowercase, not camel-case for the media query option.

I figured it would be easiest to just update the docs rather that refactor the code (e.g. -  `switch(env.dumpLineNumbers.toLowerCase())` - which would be required to maintain compatibility with previous versions).