Use subject and subject! in message

Also, add "an" to error message.

Author: bquorning

lib/rubocop/cop/rspec/empty_line_after_subject.rb

@@ -18,14 +18,15 @@ class EmptyLineAfterSubject < Cop
         extend AutoCorrector
         include RuboCop::RSpec::BlankLineSeparation
 
-        MSG = 'Add empty line after `subject`.'
+        MSG = 'Add an empty line after `%<subject>s`.'
 
         def on_block(node)
           return unless subject?(node) && !in_spec_block?(node)
           return if last_child?(node)
 
           missing_separating_line(node) do |location|
-            add_offense(location) do |corrector|
+            msg = format(MSG, subject: node.method_name)
+            add_offense(location, message: msg) do |corrector|
               corrector.insert_after(location.end, "\n")
             end
           end

spec/rubocop/cop/rspec/empty_line_after_subject_spec.rb

@@ -7,7 +7,7 @@
     expect_offense(<<-RUBY)
       RSpec.describe User do
         subject { described_class.new }
-        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add empty line after `subject`.
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add an empty line after `subject`.
         let(:params) { foo }
       end
     RUBY
@@ -25,7 +25,7 @@
     expect_offense(<<-RUBY)
       RSpec.describe User do
         subject! { described_class.new }
-        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add empty line after `subject`.
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add an empty line after `subject!`.
         let(:params) { foo }
       end
     RUBY