Fix incorrect documentation for `str::{split_at, split_at_mut}`

src/libcore/str/mod.rs

@@ -2642,7 +2642,7 @@ pub unsafe fn slice_mut_unchecked(&mut self, begin: usize, end: usize) -> &mut s
     /// # Panics
     ///
     /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
-    /// beyond the last code point of the string slice.
+    /// past the end of the last code point of the string slice.
     ///
     /// # Examples
     ///
@@ -2683,7 +2683,7 @@ pub fn split_at(&self, mid: usize) -> (&str, &str) {
     /// # Panics
     ///
     /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
-    /// beyond the last code point of the string slice.
+    /// past the end of the last code point of the string slice.
     ///
     /// # Examples
     ///