Fix `test_edge_option` failure

Follow up of 16b92eed

railties/test/generators/plugin_generator_test.rb

@@ -201,7 +201,7 @@ def test_dev_option
 
   def test_edge_option
     assert_generates_without_bundler(edge: true)
-    assert_file "Gemfile", %r{^gem\s+["']rails["'],\s+github:\s+["']#{Regexp.escape("rails/rails")}["']$}
+    assert_file "Gemfile", %r{^gem\s+["']rails["'],\s+github:\s+["']#{Regexp.escape("rails/rails")}["'],\s+branch:\s+["']#{Regexp.escape("5-2-stable")}["']$}
   end
 
   def test_generation_does_not_run_bundle_install_with_full_and_mountable