storage: fix the comment in generation.walk

storage/key_index.go

@@ -240,7 +240,7 @@ type generation struct {
 
 func (g *generation) isEmpty() bool { return g == nil || len(g.revs) == 0 }
 
-// walk walks through the revisions in the generation in ascending order.
+// walk walks through the revisions in the generation in descending order.
 // It passes the revision to the given function.
 // walk returns until: 1. it finishs walking all pairs 2. the function returns false.
 // walk returns the position at where it stopped. If it stopped after