Improve wording of static_mut_ref

Rename `static_mut_ref` lint to `static_mut_refs`.

Author: obeis

example/mini_core_hello_world.rs

@@ -99,8 +99,8 @@ fn start<T: Termination + 'static>(
 
 static mut NUM: u8 = 6 * 7;
 
-// 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 NUM_REF: &'static u8 = unsafe { &NUM };
 
 macro_rules! assert {