Fix merge request unmergeable notification email plural interpolation

app/views/notify/merge_request_unmergeable_email.html.haml

 %p
-  Merge Request #{link_to @merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request)} can no longer be merged due to the following #{pluralize(@reasons, 'reason')}:
+  Merge Request #{link_to @merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request)} can no longer be merged due to the following #{'reason'.pluralize(@reasons.count)}:
 
   %ul
     - @reasons.each do |reason|

app/views/notify/merge_request_unmergeable_email.text.haml

-Merge Request #{@merge_request.to_reference} can no longer be merged due to the following #{pluralize(@reasons, 'reason')}:
+Merge Request #{@merge_request.to_reference} can no longer be merged due to the following #{'reason'.pluralize(@reasons.count)}:
 
 - @reasons.each do |reason|
   * #{reason}

spec/mailers/notify_spec.rb

@@ -422,7 +422,7 @@ describe Notify do
           aggregate_failures do
             is_expected.to have_referable_subject(merge_request, reply: true)
             is_expected.to have_body_text(project_merge_request_path(project, merge_request))
-            is_expected.to have_body_text('reasons:')
+            is_expected.to have_body_text('following reasons:')
             reasons.each do |reason|
               is_expected.to have_body_text(reason)
             end