MM-21725 change to singular (#13634)

i18n/en.json

@@ -6296,7 +6296,7 @@
   },
   {
     "id": "store.sql_group.permanent_delete_members_by_user.app_error",
-    "translation": "Unable to remove the group members with UserID \"{{.UserId}}\""
+    "translation": "Unable to remove the group member with UserID \"{{.UserId}}\""
   },
   {
     "id": "store.sql_group.unique_constraint",