docs: fix return type of ItemSearch.items_as_dicts

[gadomski]

Related Issue(s): None

Description: In the docstring, fix the return type for items_as_dicts.

PR Checklist:

• Code is formatted
• Tests pass

[codecov-commenter]

Codecov Report

Merging #244 (78db25e) into main (a15b169) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #244   +/-   ##
=======================================
  Coverage   83.40%   83.40%           
=======================================
  Files          10       10           
  Lines         711      711           
=======================================
  Hits          593      593           
  Misses        118      118           

Impacted Files	Coverage Δ
pystac_client/item_search.py	91.90% <ø> (ø)

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[gadomski]

Fixed by #284.