Fix docs for StacIO.read/write_text

[duckontheweb]

Related Issue(s):

• Closes Discrepancy between documentation and source of StacIO read_text & write_text #764

Description:

Fixes docstrings for StacIO.read_text and StacIO.write_text to match the type annotations for the source argument.

cc: @l0b0

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #835 (3d48641) into main (a6fffa5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #835   +/-   ##
=======================================
  Coverage   94.34%   94.34%           
=======================================
  Files          83       83           
  Lines       11923    11923           
  Branches     1390     1390           
=======================================
  Hits        11249    11249           
  Misses        495      495           
  Partials      179      179           

Impacted Files	Coverage Δ
pystac/stac_io.py	87.38% <ø> (ø)

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