Fix minicore test

Author: estebank

tests/ui/atomic-from-mut-not-available.stderr

@@ -9,8 +9,9 @@ note: if you're trying to build a new `AtomicU64`, consider using `AtomicU64::ne
    = note: this error originates in the macro `atomic_int` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: there is an associated function `from` with a similar name
    |
-LL |     core::sync::atomic::AtomicU64::from(&mut 0u64);
-   |                                    ~~~~
+LL -     core::sync::atomic::AtomicU64::from_mut(&mut 0u64);
+LL +     core::sync::atomic::AtomicU64::from(&mut 0u64);
+   |
 
 error: aborting due to 1 previous error