Fix NameError in ParamUnknownKeyError formatting

[Dreamstick9]

Description:
This PR fixes a typo in awscli/argprocess.py where a NameError was raised during shorthand parsing errors because valid_key was used instead of valid_keys.

Fixes #10405

Testing Done:

• Verified locally via Python REPL that instantiating ParamUnknownKeyError now successfully formats the error string instead of crashing with a NameError.
• Ran local unit tests to ensure no regressions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.61%. Comparing base (70d3b38) to head (f99b6bb).
⚠️ Report is 42 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #10406      +/-   ##
===========================================
+ Coverage    93.42%   93.61%   +0.18%     
===========================================
  Files          211      211              
  Lines        17312    17358      +46     
===========================================
+ Hits         16174    16249      +75     
+ Misses        1138     1109      -29     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ashovlin]

Thanks for the contribution! It should go out in the next release. The v2 branch already has this fix.

[ashovlin]

This shipped in 1.45.34 today, thanks again @Dreamstick9

[Dreamstick9]

Happy to help!