Minor rewording and grammar fix

A few edits to the text here:
- the repeated use of "An Metrics" was bugging me, so I fixed them to singular.
- there were two L1 headings: the first one only seemed to deal with the Engine, so I edited it...

I also removed some whitespace to make the md file closer to the published document. This may have been foolish, as it makes it look like there have been huge changes to the document!

Feel free to disregard any changes that you don't like. And please keep up the great work!