notify: fix typo in merged_merge_request_email template

All (two) other templates with this line use 'url' rather than 'Url'.

app/views/notify/merged_merge_request_email.text.haml

 = "Merge Request ##{@merge_request.iid} was merged"
 
-Merge Request Url: #{namespace_project_merge_request_url(@merge_request.target_project.namespace, @merge_request.target_project, @merge_request)}
+Merge Request url: #{namespace_project_merge_request_url(@merge_request.target_project.namespace, @merge_request.target_project, @merge_request)}
 
 = merge_path_description(@merge_request, 'to')