raft: fix typo in raftlog

fix typo in String() method of raftlog which will misorder
the "committed" and "unstable.offset" output.

raft/log.go

@@ -65,7 +65,7 @@ func newLog(storage Storage) *raftLog {
 }
 
 func (l *raftLog) String() string {
-	return fmt.Sprintf("committed=%d, applied=%d, unstable.offset=%d, len(unstable.Entries)=%d", l.unstable.offset, l.committed, l.applied, len(l.unstable.entries))
+	return fmt.Sprintf("committed=%d, applied=%d, unstable.offset=%d, len(unstable.Entries)=%d", l.committed, l.applied, l.unstable.offset, len(l.unstable.entries))
 }
 
 // maybeAppend returns (0, false) if the entries cannot be appended. Otherwise,