English language proofreading up to 12.1 (including)

I'm done up to 12.1, this is probably the last pull request for the year, as I'll be away at 31c3, I'll continue in January.
Thanks for your comments! I fixed everything: https://github.com/dennis714/RE-for-beginners/commit/689c674915f4f79acfa1219fd166dccd370df060