docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/_themes/README.rst
• docs/caching.rst
• imagekit/files.py
• imagekit/forms/fields.py
• imagekit/specs/sourcegroups.py
• tests/test_optimistic_strategy.py

Fixes:

• Should read underlying rather than underlaying.
• Should read truthiness rather than thruthiness.
• Should read separate rather than seprate.
• Should read response rather than responce.
• Should read represents rather than repesents.
• Should read perform rather than peform.
• Should read directly rather than dirrectly.
• Should read derivative rather than drivative.

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