Update orbax_upgrade_guide.rst for async checkpointing usage examples

[kaushaladiti-2802]

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What does this PR do?

Fixes # (issue)

Checklist

• [ ] This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• [ ] This change is discussed in a Github issue/ discussion (please add a link).
• [ ] The documentation and docstrings adhere to the documentation guidelines.
• [ ] This change includes necessary high-coverage tests. (No quality testing = no merge!)

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 0.00%. Comparing base (31adb00) to head (4910005). Report is 72 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #4036    +/-   ##
======================================
  Coverage   0.00%   0.00%            
======================================
  Files        106     107     +1     
  Lines      13582   13824   +242     
======================================
- Misses     13582   13824   +242     

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