Fixed copy+paste error in comment

Co-authored-by: Mara Bos <[email protected]>

Author: EliasHolzmann

library/core/src/fmt/mod.rs

@@ -261,7 +261,7 @@ pub enum Sign {
 pub enum DebugAsHex {
     /// Use lower-case hexadecimal integers for the `Debug` trait (like [the `x?` type](../../std/fmt/index.html#formatting-traits)).
     Lower,
-    /// Use upper-case hexadecimal integers for the `Debug` trait (like [the `x?` type](../../std/fmt/index.html#formatting-traits)).
+    /// Use upper-case hexadecimal integers for the `Debug` trait (like [the `X?` type](../../std/fmt/index.html#formatting-traits)).
     Upper,
 }