typo fix

src/libcollections/vec.rs

@@ -1213,7 +1213,7 @@ impl<T: Clone> Vec<T> {
         unsafe {
             let mut ptr = self.as_mut_ptr().offset(self.len() as isize);
             // Use SetLenOnDrop to work around bug where compiler
-            // may not realize the store through `ptr` trough self.set_len()
+            // may not realize the store through `ptr` through self.set_len()
             // don't alias.
             let mut local_len = SetLenOnDrop::new(&mut self.len);