fixed documentation problems related to "its" and "it's"

[Matthias Lang]

Hi,

I went through all the .xml files in the git repository and fixed every
wrong use of "its" versus "it's" I could find. And vice versa. 

 git://github.com/matthiasl/otp.git documentation_apostrophe_fix

(about 40 changes in about 25 files)

Matt