Fix the test

Author: mark-i-m

src/test/ui/feature-gate-macro_at_most_once_rep.rs

@@ -12,7 +12,7 @@
 // gate is not used.
 
 macro_rules! m { ($(a)?) => {} }
-//~^ ERROR Using the `?` macro Kleene operator for "at most one" repetition is unstable (see issue #48075)
+//~^ ERROR Using the `?` macro Kleene operator for "at most one" repetition is unstable
 
 fn main() {
     m!();

src/test/ui/feature-gate-macro_at_most_once_rep.stderr

@@ -1,4 +1,4 @@
-error[E0658]: Using the `?` macro Kleene operator for "at most one" repetition is unstable
+error[E0658]: Using the `?` macro Kleene operator for "at most one" repetition is unstable (see issue #48075)
   --> $DIR/feature-gate-macro_at_most_once_rep.rs:14:20
    |
 14 | macro_rules! m { ($(a)?) => {} }