DoubleExtensions: Fix typo, was DoubleExtentions

[ScarletKuro]

Description

Just a typo in a static class name

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.97%. Comparing base (b59465c) to head (73973a3).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8473      +/-   ##
==========================================
- Coverage   89.05%   88.97%   -0.09%     
==========================================
  Files         411      411              
  Lines       12185    12185              
  Branches     2433     2433              
==========================================
- Hits        10851    10841      -10     
- Misses        800      806       +6     
- Partials      534      538       +4     

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[henon]

Added to the v7.0.0 migration guide.