Update vec-drain.md

missed comma

src/vec-drain.md

@@ -11,7 +11,7 @@ struct Drain<'a, T: 'a> {
     // Need to bound the lifetime here, so we do it with `&'a mut Vec<T>`
     // because that's semantically what we contain. We're "just" calling
     // `pop()` and `remove(0)`.
-    vec: PhantomData<&'a mut Vec<T>>
+    vec: PhantomData<&'a mut Vec<T>>,
     start: *const T,
     end: *const T,
 }