Improve hint for as_ref_mut

Author: Mikael Frosthage

info.toml

@@ -1135,4 +1135,4 @@ name = "as_ref_mut"
 path = "exercises/conversions/as_ref_mut.rs"
 mode = "test"
 hint = """
-Add AsRef<str> as a trait bound to the functions."""
+Add AsRef<str> or AsMut<u32> as a trait bound to the functions."""