chore: fix branch of rule docs link URL to main

[ota-meshi]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• New rule
• Changes an existing rule
• Add autofixing to a rule
• Other, please explain:

What changes did you make? (Give an overview)

This PR fixes the rule's meta data to change the branch that the rule's doc URL links from master to main.

It's fine because it redirects even before the change, but it doesn't redirect after this change.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (302bcb4) compared to base (71e53a0).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #381   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines          626       626           
  Branches       242       242           
=========================================
  Hits           626       626           

Impacted Files	Coverage Δ
rules/lib/get-docs-url.js	100.00% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 6.1.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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