Update mod.rs

Author: vwkd

library/core/src/str/mod.rs

@@ -806,7 +806,7 @@ impl str {
     /// assert_eq!(Some((0, 'y')), char_indices.next()); // not (0, 'y̆')
     /// assert_eq!(Some((1, '\u{0306}')), char_indices.next());
     ///
-    /// // note the 3 here - the last character took up two bytes
+    /// // note the 3 here - the previous character took up two bytes
     /// assert_eq!(Some((3, 'e')), char_indices.next());
     /// assert_eq!(Some((4, 's')), char_indices.next());
     ///