Update wording on how to issue a pull request [ci skip]
Update wording on how to issue a pull request [ci skip]

Updated the wording https://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#issue-a-pull-request to better match the current state of Github's UI. I also made it more explicit which branch new PRs should go to, based on some feedback [here](https://github.com/rails/rails/pull/42412#issuecomment-856004305).
Envek/rails,rails/rails,esparta/rails,lcreid/rails,EmmaB/rails-1,yahonda/rails,Edouard-chin/rails,kmcphillips/rails,BlakeWilliams/rails,MSP-Greg/rails,georgeclaghorn/rails,georgeclaghorn/rails,joonyou/rails,esparta/rails,lcreid/rails,Vasfed/rails,Envek/rails,Vasfed/rails,BlakeWilliams/rails,Edouard-chin/rails,Envek/rails,lcreid/rails,yawboakye/rails,Stellenticket/rails,georgeclaghorn/rails,kddeisz/rails,Edouard-chin/rails,yawboakye/rails,rails/rails,joonyou/rails,Erol/rails,MSP-Greg/rails,joonyou/rails,rails/rails,yawboakye/rails,Edouard-chin/rails,EmmaB/rails-1,esparta/rails,yahonda/rails,Vasfed/rails,Envek/rails,Erol/rails,kddeisz/rails,kmcphillips/rails,Erol/rails,Stellenticket/rails,joonyou/rails,rails/rails,kddeisz/rails,MSP-Greg/rails,EmmaB/rails-1,kmcphillips/rails,kddeisz/rails,georgeclaghorn/rails,MSP-Greg/rails,kmcphillips/rails,Vasfed/rails,esparta/rails,Stellenticket/rails,Erol/rails,yahonda/rails,BlakeWilliams/rails,BlakeWilliams/rails,lcreid/rails,yawboakye/rails,Stellenticket/rails,yahonda/rails