Improve wording for approve comment

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/services/system_note_service.rb

@@ -262,11 +262,11 @@ class SystemNoteService
   #
   # Example Note text:
   #
-  #   "Approved by @rspeicher"
+  #   "Approved this merge request"
   #
   # Returns the created Note object
   def self.approve_mr(noteable, user)
-    body = "Approved by @#{user.username}"
+    body = "Approved this merge request"
 
     create_note(noteable: noteable, project: noteable.project, author: user, note: body)
   end

spec/services/system_note_service_spec.rb

@@ -449,9 +449,9 @@ describe SystemNoteService do
 
     it_behaves_like 'a system note'
 
-    context 'when assignee added' do
+    context 'when merge request approved' do
       it 'sets the note text' do
-        expect(subject.note).to eq "Approved by @#{author.username}"
+        expect(subject.note).to eq "Approved this merge request"
       end
     end
   end