wal: fix wal doc

wal/doc.go

@@ -48,7 +48,7 @@ Cut issues 0x10 entries with incremental index later then the file will be calle
 
 At a later time a WAL can be opened at a particular raft index:
 
-	w, err := wal.OpenAtIndex("/var/lib/etcd", 0)
+	w, err := wal.Open("/var/lib/etcd", 0)
 	...
 
 The raft index must have been written to the WAL. When opening without a