Add missing RSpec references

Author: JonRowe

spec/rspec/rails/example/channel_example_group_spec.rb

@@ -2,7 +2,7 @@
 require "rspec/rails/feature_check"
 
 module RSpec::Rails
-  describe ChannelExampleGroup do
+  RSpec.describe ChannelExampleGroup do
     if RSpec::Rails::FeatureCheck.has_action_cable_testing?
       it_behaves_like "an rspec-rails example group mixin", :channel,
         './spec/channels/', '.\\spec\\channels\\'

spec/rspec/rails/example/helper_example_group_spec.rb

@@ -1,7 +1,7 @@
 require "spec_helper"
 
 module RSpec::Rails
-  describe HelperExampleGroup do
+  RSpec.describe HelperExampleGroup do
     module ::FoosHelper
       class InternalClass
       end

spec/rspec/rails/example/mailbox_example_group_spec.rb

@@ -15,7 +15,7 @@ def receive(*)
 
 module RSpec
   module Rails
-    describe MailboxExampleGroup, :skip => !RSpec::Rails::FeatureCheck.has_action_mailbox? do
+    RSpec.describe MailboxExampleGroup, :skip => !RSpec::Rails::FeatureCheck.has_action_mailbox? do
       it_behaves_like "an rspec-rails example group mixin", :mailbox,
                       './spec/mailboxes/', '.\\spec\\mailboxes\\'