Fix the documentation of Option::replace

Author: Clément RENAULT

src/libcore/option.rs

@@ -848,7 +848,7 @@ impl<T> Option<T> {
 
     /// Replaces the actual value in the option by the value given in parameter,
     /// returning the old value if present,
-    /// leaving a `Some` in its place without deinitializing either one.
+    /// leaving a [`Some`] in its place without deinitializing either one.
     ///
     /// [`Some`]: #variant.Some
     ///