fix: date format error in get_user_name_change

[Lucino772]

Description

This PR fixes an issue in the get_user_name_change function.

Fixes #71

Type of change

• 🪳 Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (5ee8f92) 79.02% compared to head (3b97d2d) 79.02%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   79.02%   79.02%           
=======================================
  Files          25       25           
  Lines         820      820           
  Branches       79       79           
=======================================
  Hits          648      648           
  Misses        148      148           
  Partials       24       24           

Impacted Files	Coverage Δ
mojang/api/session.py	100.00% <100.00%> (ø)

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