cfg-attr-multi: Change issue number to actual tracking issue

Author: Havvy

src/doc/unstable-book/src/language-features/cfg-attr-multi.md

@@ -1,9 +1,9 @@
 # `cfg_attr_multi`
 
-The tracking issue for this feature is: [#555666]
+The tracking issue for this feature is: [#54881]
 The RFC for this feature is: [#2539]
 
-[#555666]: https://github.com/rust-lang/rust/issues/555666
+[#54881]: https://github.com/rust-lang/rust/issues/54881
 [#2539]: https://github.com/rust-lang/rfcs/pull/2539
 
 ------------------------

src/libsyntax/feature_gate.rs

@@ -517,7 +517,7 @@ declare_features! (
     (active, impl_trait_in_bindings, "1.30.0", Some(34511), None),
 
     // #[cfg_attr(predicate, multiple, attributes, here)]
-    (active, cfg_attr_multi, "1.31.0", Some(555666), None),
+    (active, cfg_attr_multi, "1.31.0", Some(54881), None),
 );
 
 declare_features! (

src/test/ui/feature-gates/feature-gate-cfg-attr-multi-1.stderr

@@ -1,4 +1,4 @@
-error[E0658]: cfg_attr with zero or more than one attributes is experimental (see issue #555666)
+error[E0658]: cfg_attr with zero or more than one attributes is experimental (see issue #54881)
   --> $DIR/feature-gate-cfg-attr-multi-1.rs:3:1
    |
 LL | #![cfg_attr(all(), warn(nonstandard_style), allow(unused_attributes))]

src/test/ui/feature-gates/feature-gate-cfg-attr-multi-2.stderr

@@ -1,4 +1,4 @@
-error[E0658]: cfg_attr with zero or more than one attributes is experimental (see issue #555666)
+error[E0658]: cfg_attr with zero or more than one attributes is experimental (see issue #54881)
   --> $DIR/feature-gate-cfg-attr-multi-2.rs:1:1
    |
 LL | #![cfg_attr(all(),)]