add back replacement character

Author: HTGAzureX1212

library/alloc/src/string.rs

@@ -587,7 +587,7 @@ impl String {
     /// between the two. Not all byte slices are valid strings, however: strings
     /// are required to be valid UTF-8. During this conversion,
     /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
-    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD], which looks like this:  
+    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD], which looks like this: �
     ///
     /// [byteslice]: prim@slice
     /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER