Remove stabilized feature gate in doctest

Author: SimonSapin

src/libcore/convert.rs

@@ -415,7 +415,6 @@ pub trait TryInto<T>: Sized {
 /// As described, [`i32`] implements `TryFrom<i64>`:
 ///
 /// ```
-/// #![feature(try_from)]
 /// use std::convert::TryFrom;
 ///
 /// let big_number = 1_000_000_000_000i64;