Client.search: fix parameter documentation

[adamjstewart]

Description:

Fixes minor typo in https://pystac-client.readthedocs.io/en/stable/api.html#pystac_client.Client.search

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.13%. Comparing base (21435b0) to head (f8cb85f).
Report is 119 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #789      +/-   ##
==========================================
+ Coverage   93.43%   94.13%   +0.69%     
==========================================
  Files          13       15       +2     
  Lines         990     1210     +220     
==========================================
+ Hits          925     1139     +214     
- Misses         65       71       +6     

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