correct mutability

Author: KGrewal1

Diff for: library/core/src/str/mod.rs

@@ -785,8 +785,8 @@ impl str {
         // SAFETY: caller guarantees `mid` is on a char boundary.
         unsafe {
             (
-                from_utf8_unchecked_mut(slice::from_raw_parts(ptr, mid)),
-                from_utf8_unchecked_mut(slice::from_raw_parts(ptr.add(mid), len - mid)),
+                from_utf8_unchecked(slice::from_raw_parts(ptr, mid)),
+                from_utf8_unchecked(slice::from_raw_parts(ptr.add(mid), len - mid)),
             )
         }
     }