Fix intrasite links substitions in content

The Content.__eq__ method would indirectly call _update_content too
soon, resulting in failed intrasite links substitution
This effectively reverts fd779267000ac539ee0a9ba5856d103fbbc7cd7c
for pelican/contents.py, it was unnecessary anyways.
Thanks to Strom for finding this.
Fix typo in command-line option description.