Set branch references to main

Author: JonRowe

source/contributing.html.slim

@@ -12,6 +12,6 @@ section
       | See 
       = link_to 'rspec-dev', 'https://github.com/rspec/rspec-dev'
       |  for how to develop on and test RSpec, as well as any 
-      = link_to 'DEV-README.md', 'https://github.com/rspec/rspec-core/blob/master/DEV-README.md'
+      = link_to 'DEV-README.md', 'https://github.com/rspec/rspec-core/blob/main/DEV-README.md'
       |  that may be in that repository.
       = partial "repos"