Remove unnecessary mut in iterator.find_map documentation example, Re…

…lates to rust-lang#49098

src/libcore/iter/iterator.rs

@@ -1857,7 +1857,7 @@ pub trait Iterator {
     /// ```
     /// let a = ["lol", "NaN", "2", "5"];
     ///
-    /// let mut first_number = a.iter().find_map(|s| s.parse().ok());
+    /// let first_number = a.iter().find_map(|s| s.parse().ok());
     ///
     /// assert_eq!(first_number, Some(2));
     /// ```