fix typo "this files" -> "this file"

app/views/projects/diffs/_file.html.haml

@@ -5,7 +5,7 @@
     - unless diff_file.submodule?
       .file-actions.hidden-xs
         - if blob_text_viewable?(blob)
-          = link_to '#', class: 'js-toggle-diff-comments btn active has-tooltip btn-file-option', title: "Toggle comments for this files", disabled: @diff_notes_disabled do
+          = link_to '#', class: 'js-toggle-diff-comments btn active has-tooltip btn-file-option', title: "Toggle comments for this file", disabled: @diff_notes_disabled do
             = icon('comment')
           \