Rollup merge of rust-lang#113750 - nipzu:italicize-sort-complexity, r=workingjubilee

Add missing italicization to `sort_unstable_by_key` complexity

Other methods like `sort_by_key` already had `m` italicized.

Author: matthiaskrgr

library/core/src/slice/mod.rs

@@ -2957,7 +2957,7 @@ impl<T> [T] {
     /// elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place
-    /// (i.e., does not allocate), and *O*(m \* *n* \* log(*n*)) worst-case, where the key function is
+    /// (i.e., does not allocate), and *O*(*m* \* *n* \* log(*n*)) worst-case, where the key function is
     /// *O*(*m*).
     ///
     /// # Current implementation