docs(feature-flags): correct docs and typing

[michaelbrewer]

Issue #, if available:

Description of changes:

• Demark optional value
• Correct doctstring indents for pdoc3
• Correct example code type to python

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #588 (d092123) into develop (92d4a6d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #588   +/-   ##
========================================
  Coverage    99.95%   99.95%           
========================================
  Files          113      113           
  Lines         4533     4533           
  Branches       245      245           
========================================
  Hits          4531     4531           
  Partials         2        2           

Impacted Files	Coverage Δ
...da_powertools/utilities/feature_flags/appconfig.py	100.00% <ø> (ø)
..._lambda_powertools/utilities/feature_flags/base.py	100.00% <ø> (ø)
...owertools/utilities/feature_flags/feature_flags.py	100.00% <ø> (ø)
...ambda_powertools/utilities/feature_flags/schema.py	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 92d4a6d...d092123. Read the comment docs.