runtime: fix typo in mapextra comment

Change-Id: Idbd8a1b5bfeb1c23c86cef0697cf0380900e95f3
GitHub-Last-Rev: a8c2b27046582c4eef932a8502826a3b23b8dab3
GitHub-Pull-Request: golang/go#26175
Reviewed-on: https://go-review.googlesource.com/121821Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/map.go

@@ -126,7 +126,7 @@ type mapextra struct {
 	// If both key and value do not contain pointers and are inline, then we mark bucket
 	// type as containing no pointers. This avoids scanning such maps.
 	// However, bmap.overflow is a pointer. In order to keep overflow buckets
-	// alive, we store pointers to all overflow buckets in hmap.overflow and h.map.oldoverflow.
+	// alive, we store pointers to all overflow buckets in hmap.extra.overflow and hmap.extra.oldoverflow.
 	// overflow and oldoverflow are only used if key and value do not contain pointers.
 	// overflow contains overflow buckets for hmap.buckets.
 	// oldoverflow contains overflow buckets for hmap.oldbuckets.