Simplify instructions for gems installations from git repository

README.md

@@ -45,15 +45,8 @@ See the [`7-1-maintenance` branch on Github](https://github.com/rspec/rspec-rail
    end
 
    # Or, run against the main branch
-   # (requires main-branch versions of all related RSpec libraries)
    group :development, :test do
-     %w[rspec-core rspec-expectations rspec-mocks rspec-rails rspec-support].each do |lib|
-       if lib == 'rspec-rails'
-         gem lib, git: "https://github.com/rspec/rspec-rails"
-       else
-         gem lib, git: "https://github.com/rspec/rspec", glob: "#{lib}/#{lib}.gemspec"
-       end
-     end
+     gem 'rspec-rails', git: 'https://github.com/rspec/rspec-rails'
    end
    ```