Fix name of puma's example config file in README.md

What does this MR do and why?

This MR fixes name of the file in README.md: config/puma.rb.example.development => config/puma.example.development.rb

https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/config/puma.rb.example.development does not exitst on 'master'.

See https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/puma.example.development.rb

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