[FIX] change "grey" to "gray" in doc/visual_testing/reporter_visual_inspection_suite.py for older Matplotlib versions

[michellewang]

• Closes [BUG] Matplotlib colormap not recognized in reporter_visual_inspection_suite.py due to British/Canadian spelling #4351

Changes proposed in this pull request:

• Change "grey" to "gray"

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.07%. Comparing base (abb80ff) to head (b63101b).
Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4353      +/-   ##
==========================================
+ Coverage   91.85%   92.07%   +0.21%     
==========================================
  Files         144      143       -1     
  Lines       16419    16496      +77     
  Branches     3434     3463      +29     
==========================================
+ Hits        15082    15188     +106     
+ Misses        792      760      -32     
- Partials      545      548       +3     

Flag	Coverage Δ
macos-latest_3.10_test_plotting	91.94% <ø> (?)
macos-latest_3.11_test_plotting	?
ubuntu-latest_3.10_test_plotting	91.94% <ø> (+0.09%)	⬆️
ubuntu-latest_3.11_test_plotting	91.94% <ø> (?)
ubuntu-latest_3.12_test_plotting	91.94% <ø> (?)
ubuntu-latest_3.8_test_min	68.83% <ø> (?)
ubuntu-latest_3.9_test_plotting	91.91% <ø> (?)
windows-latest_3.11_test_plotting	91.92% <ø> (?)
windows-latest_3.12_test_plotting	91.92% <ø> (?)

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[man-shu]

Looks good! Thanks!