Improve wording of static_mut_ref

Rename `static_mut_ref` lint to `static_mut_refs`.

src/tools/miri/tests/fail/tls/tls_static_dealloc.rs

@@ -1,8 +1,8 @@
 //! Ensure that thread-local statics get deallocated when the thread dies.
 
 #![feature(thread_local)]
-// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_ref` lint
-#![allow(static_mut_ref)]
+// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_refs` lint
+#![allow(static_mut_refs)]
 
 #[thread_local]
 static mut TLS: u8 = 0;

src/tools/miri/tests/pass/static_mut.rs

@@ -1,7 +1,7 @@
 static mut FOO: i32 = 42;
 
-// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_ref` lint
-#[allow(static_mut_ref)]
+// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_refs` lint
+#[allow(static_mut_refs)]
 static BAR: Foo = Foo(unsafe { &FOO as *const _ });
 
 #[allow(dead_code)]

src/tools/miri/tests/pass/tls/tls_static.rs

@@ -8,8 +8,8 @@
 //! test, we also check that thread-locals act as per-thread statics.
 
 #![feature(thread_local)]
-// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_ref` lint
-#![allow(static_mut_ref)]
+// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_refs` lint
+#![allow(static_mut_refs)]
 
 use std::thread;