Fix typo in alloc vec comment

Author: pickfire

library/alloc/src/vec.rs

@@ -2281,7 +2281,7 @@ where
 
         // use try-fold since
         // - it vectorizes better for some iterator adapters
-        // - unlike most internal iteration methods methods it only takes a &mut self
+        // - unlike most internal iteration methods, it only takes a &mut self
         // - it lets us thread the write pointer through its innards and get it back in the end
         let sink = InPlaceDrop { inner: dst_buf, dst: dst_buf };
         let sink = iterator