docs: Fix a few typos

[timgates42]

There are small typos in:

• CONTRIBUTING.rst
• README.rst
• docs/custom_usage.rst
• docs/reference/backends.rst
• docs/usage.rst

Fixes:

• Should read satisfied rather than satisifed.
• Should read responsible rather than responsbile.
• Should read relevant rather than releveant.
• Should read precedes rather than preceds.
• Should read against rather than agains.

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

[bennylope]

Thank you! These kinds of changes may seem small but in aggregate they make a difference.