Remove documentation about standalone git tag

Specify a git source by itself is no longer supported in the latest version of Bundler.

      You can no longer specify a git source by itself. Instead, 
      either use the :git option on a gem, or specify the gems that 
      bundler should find in the git source by passing a block to 
      the git method, like: 
      
        git 'git://github.com/rails/rails.git' do
          gem 'rails'
        end