Change doc link

Author: WilliamVenner

library/alloc/src/string.rs

@@ -1987,7 +1987,9 @@ impl String {
 
     /// Replaces the leftmost occurrence of a pattern with another string, in-place.
     ///
-    /// This method should be preferred over [`String::replacen(..., 1)`](str::replacen) as it can use the `String`'s existing capacity to prevent a reallocation if sufficient space is available.
+    /// This method should be preferred over [`str::replacen(..., 1)`] as it can use the `String`'s existing capacity to prevent a reallocation if sufficient space is available.
+    ///
+    /// [`str::replacen(..., 1)`]: str::replacen
     ///
     /// # Examples
     ///