A few things with documentation

[Isaac Bennetch]

Hello, and thanks for the great work on the great project. I use it all
the time.

I've noticed a few things with the documentation that don't quite make
enough sense to me to actually fix on my own (or may be intentional),
but are worth reporting. These each refer to INSTALL.rst.

1) There's a link on line 81 for "official git repository" that's broken.

2) In several places throughout the documentation, the "spaetz" github
repository is referred to. However, going to that repository claims that
this is "my private development sandbox", so the links should probably
be updated to "OfflineIMAP" instead.

3) On line 111, the text "You tofflineimap to invoke the program" makes
no sense. Best I can figure was meant was "You type `./offlineimap.py`to
invoke the program."

Kind regards,
~isaac