add back replacement character

Author: HTGAzureX1212

library/alloc/src/string.rs

@@ -2150,7 +2150,7 @@ impl FromUtf8Error {
     /// let input: Vec<u8> = b"Hello \xF0\x90\x80World".into();
     /// let output = String::from_utf8(input).unwrap_or_else(|e| e.into_utf8_lossy());
     ///
-    /// assert_eq!(String::from("Hello  World"), output);
+    /// assert_eq!(String::from("Hello �World"), output);
     /// ```
     #[must_use]
     #[cfg(not(no_global_oom_handling))]