Do not show credentials in generators help

Since credentials generator is executed via the credentials command and
does not need to be executed directly, so it is not necessary to show it in
help.

railties/lib/rails/generators.rb

@@ -218,6 +218,7 @@ module Rails
         rails.delete("app")
         rails.delete("plugin")
         rails.delete("encrypted_secrets")
+        rails.delete("credentials")
 
         hidden_namespaces.each { |n| groups.delete(n.to_s) }
 

railties/test/application/generators_test.rb

@@ -188,10 +188,11 @@ module ApplicationTests
       Rails::Command.send(:remove_const, "APP_PATH")
     end
 
-    test "help does not show hidden namespaces" do
+    test "help does not show hidden namespaces and hidden commands" do
       FileUtils.cd(rails_root) do
         output = rails("generate", "--help")
         assert_no_match "active_record:migration", output
+        assert_no_match "credentials", output
 
         output = rails("destroy", "--help")
         assert_no_match "active_record:migration", output