docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/contributing.rst
• docs/customuser.rst
• docs/install.rst
• docs/notifications.rst
• docs/settings.rst
• docs/templatetags.rst
• docs/testing.rst
• docs/updates.rst
• pybb/compat.py
• pybb/forms.py
• pybb/models.py
• pybb/tests.py

Fixes:

• Should read splitted rather than splited.
• Should read environment rather than environnement.
• Should read overridden rather than overrided.
• Should read example rather than exemple.
• Should read standard rather than standrard.
• Should read should rather than shoud.
• Should read performance rather than perfomance.
• Should read inherit rather than inherite.
• Should read assignment rather than assigment.
• Should read symbols rather than sybmols.
• Should read subscribers rather than subcribers.
• Should read requirements rather than requirments.
• Should read recommended rather than recommendet.
• Should read recommended rather than recommanded.
• Should read privileges rather than privilesges.
• Should read permission rather than permisssion.
• Should read optional rather than optionnal.
• Should read itself rather than itselt.
• Should read default rather than dafault.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[lampslave]

Thank you!