Change contraction "it's" to possessive "its"

[PeterWhittaker]

The last sentence had a contraction of "it is" where the possessive "its" should have been used.

[hoijui]

We applied this and three more Pull-Request in our fork of this repo,
which we plan to maintain, and are happy to accept others as maintainers in:

https://github.com/osegermany/git-internals-pdf/