Fix a couple of its/it's errors.

[direvus]

It was made slightly tricky to check for all instances of this, because there are a mixture of ASCII apostrophes with U+2019 "right single quotation mark" in the documents. But I think I managed to "catch 'em all". I didn't find any instances of U+02BC "modifier letter apostrophe".

Thanks for the guide! I hope you find this patch helpful.