Update template URLs to use main

[misscoded]

Summary

Template URLs for Contributing Guidelines is currently pointing to master, resulting in a redirect. Switch to main.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #667 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #667   +/-   ##
=======================================
  Coverage   84.41%   84.41%           
=======================================
  Files           8        8           
  Lines         693      693           
  Branches      206      206           
=======================================
  Hits          585      585           
  Misses         71       71           
  Partials       37       37           

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