Enable feature in doctest.

Author: calder

library/core/src/primitive_docs.rs

@@ -1335,6 +1335,7 @@ mod prim_f16 {}
 /// For example:
 ///
 /// ```
+/// # #![feature(float_algebraic)]
 /// # let mut x: f32 = 12.0;
 /// # let a: f32 = 34.0;
 /// # let b: f32 = 56.0;