Fix _ParentType annotation

[dcharatan]

Fixes definition of _ParentType so that runtime type checkers work with nested Flax modules.

What does this PR do?

Fixes #3756. The parents that _ParentType refers to are instances of classes, not the classes themselves, so type[Module] and type[_Sentinel] should become Module and _Sentinel respectively.

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 here

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (31adb00) to head (967353f).
Report is 119 commits behind head on main.

Files	Patch %	Lines
flax/linen/module.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #4120    +/-   ##
======================================
  Coverage   0.00%   0.00%            
======================================
  Files        106     109     +3     
  Lines      13582   14216   +634     
======================================
- Misses     13582   14216   +634     

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