Update src/libcore/option.rs

Co-authored-by: Steve Klabnik <steve@steveklabnik.com>

src/libcore/option.rs

@@ -1405,6 +1405,7 @@ impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
     /// ```
     /// let mut s = Some(String::from("Hello"));
     /// let o: Option<&mut String> = Option::from(&mut s);
+    ///
     /// match o {
     ///     Some(t) => *t = String::from("Hello, Rustaceans!"),
     ///     None => (),