Updated spelling, grammar and missing words.

I updated the grammar again. Also some spelling mistakes and one or two missing words.

I also made the section sect6_developers roughly say "Tox is a program created by numerous persons around the globe with the intention that the user should be in control of the program and not the other way around. Therefore Tox is completely open source and free software." since I think the previous message in that section didn't go well together with the paragraph "Free Software".

--Zarkann