docs: correct img_res_key references in spatial plotting

[Georicl]

Description

Correct three docstring references from img_key to img_res_key in spatial plotting documentation, including the shared scale_factor description, to match the existing API.

How has this been tested?

• Passed pre-commit checks for both changed files.
• Built the complete documentation with Sphinx warnings treated as errors (-W); zero warnings.
• Verified the generated spatial_scatter and spatial_segment API pages and their parameter signatures.

Closes

Closes #884.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.44%. Comparing base (8286274) to head (73e4344).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1290   +/-   ##
=======================================
  Coverage   78.44%   78.44%           
=======================================
  Files          63       63           
  Lines        9532     9532           
  Branches     1594     1594           
=======================================
  Hits         7477     7477           
  Misses       1489     1489           
  Partials      566      566           

Files with missing lines	Coverage Δ
src/squidpy/_docs.py	95.34% <ø> (ø)
src/squidpy/pl/_spatial.py	92.85% <ø> (ø)

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