Rm hiding feature gate & add 1 more example

Author: JulianKnodt

library/core/src/array/mod.rs

@@ -374,7 +374,7 @@ impl<T, const N: usize> [T; N] {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(array_map)]
+    /// #![feature(array_map)]
     /// let x = [1, 2, 3];
     /// let y = x.map(|v| v + 1);
     /// assert_eq!(y, [2, 3, 4]);
@@ -383,6 +383,10 @@ impl<T, const N: usize> [T; N] {
     /// let mut temp = 0;
     /// let y = x.map(|v| { temp += 1; v * temp });
     /// assert_eq!(y, [1, 4, 9]);
+    ///
+    /// let x = ["Ferris", "Bueller's", "Day", "Off"];
+    /// let y = x.map(|v| v.len());
+    /// assert_eq!(y, [6, 9, 3, 3]);
     /// ```
     #[unstable(feature = "array_map", issue = "75243")]
     pub fn map<F, U>(self, mut f: F) -> [U; N]