raft: fix var name in comment

raft/storage.go

@@ -25,7 +25,7 @@ import (
 // index is unavailable because it predates the last snapshot.
 var ErrCompacted = errors.New("requested index is unavailable due to compaction")
 
-// ErrOutOfDataSnap is returned by Storage.CreateSnapshot when a requested
+// ErrSnapOutOfDate is returned by Storage.CreateSnapshot when a requested
 // index is older than the existing snapshot.
 var ErrSnapOutOfDate = errors.New("requested index is older than the existing snapshot")