Fix: locale on reviewer sidebar

* Fix wrong translation token used in reviewer sidebar.

Changelog: fixed

app/views/shared/issuable/_sidebar_reviewers.html.haml

@@ -2,7 +2,7 @@
 
 #js-vue-sidebar-reviewers{ data: { field: issuable_type, signed_in: signed_in } }
   .title.hide-collapsed
-    = _('Reviewer')
+    = _('Reviewers')
     = gl_loading_icon(inline: true)
 
 .selectbox.hide-collapsed