copy: correct misuse of quote in diagnostic

* src/copy.c (copy_reg): Multiple uses of quote (s) in an
argument list is erroneous.  Use quote_n, instead.

src/copy.c

@@ -975,7 +975,7 @@ copy_reg (char const *src_name, char const *dst_name,
           if (!clone_ok)
             {
               error (0, errno, _("failed to clone %s from %s"),
-                     quote (dst_name), quote (src_name));
+                     quote_n (0, dst_name), quote_1 (1, src_name));
               return_val = false;
               goto close_src_and_dst_desc;
             }