core::convert::identity: fix issue number to #53500

Author: Centril

src/libcore/convert.rs

@@ -103,7 +103,7 @@
 /// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
 /// assert_eq!(vec![1, 3], filtered);
 /// ```
-#[unstable(feature = "convert_id", issue = "0")]
+#[unstable(feature = "convert_id", issue = "53500")]
 #[rustc_const_unstable(feature = "const_convert_id")]
 #[inline]
 pub const fn identity<T>(x: T) -> T { x }