Stabilize field init shorthand

Closes rust-lang#37340.

Author: est31

reference.md

@@ -2825,7 +2825,6 @@ This allows a compact syntax with less duplication.
 Example:
 
 ```
-# #![feature(field_init_shorthand)]
 # struct Point3d { x: i32, y: i32, z: i32 }
 # let x = 0;
 # let y_value = 0;