grammar fixes

Hi,

I fixed English grammar in a couple of places I noticed while reading the documentation. The changes should be simple fixes.

I didn't do any stylistic changes, where things are unclear or a bit hard to understand. I might write something if you would like to use it. As a disclaimer, I am not a native English speaker, so it probably won't be 100% correct grammar. For instance, I don't know where commas should be placed, but I believe I can help.

Ivan