Simplify instructions for gems installations from git repository (#2828)

Author: rubysamurai

README.md

@@ -46,11 +46,8 @@ require the latest unstable features.
    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|
-       gem lib, git: "https://github.com/rspec/#{lib}.git", branch: 'main'
-     end
+     gem 'rspec-rails', git: 'https://github.com/rspec/rspec-rails'
    end
    ```