fix: Dedent attribute docstrings' content.

[DanCardin]

Fixes #273

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.39%. Comparing base (d68b762) to head (8e5f419).

Files with missing lines	Patch %	Lines
src/cappa/help.py	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
- Coverage   99.45%   99.39%   -0.07%     
==========================================
  Files          28       28              
  Lines        2960     2970      +10     
  Branches      671      672       +1     
==========================================
+ Hits         2944     2952       +8     
- Misses         12       13       +1     
- Partials        4        5       +1     

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