Make From::from example more idiomatic / simpler

src/libcore/convert.rs

@@ -83,10 +83,8 @@ pub trait Into<T>: Sized {
 /// `String` implements `From<&str>`:
 ///
 /// ```
-/// let s = "hello";
 /// let string = "hello".to_string();
-///
-/// let other_string: String = From::from(s);
+/// let other_string = String::from("hello");
 ///
 /// assert_eq!(string, other_string);
 /// ```