container/list: fix typo

R=golang-dev, fullung, dave, minux.ma
CC=golang-dev
https://golang.org/cl/6682046

src/pkg/container/list/list.go

@@ -176,7 +176,7 @@ func (l *List) MoveToBack(e *Element) {
 	l.insert(l.remove(e), l.root.prev)
 }
 
-// PuchBackList inserts a copy of an other list at the back of list l.
+// PushBackList inserts a copy of an other list at the back of list l.
 // The lists l and other may be the same.
 func (l *List) PushBackList(other *List) {
 	l.lazyInit()