Typo fix in gauche.serializer

output-serializer?(gauche.serializer) had a typo.

```
$ ./gosh -ftest -ugauche.test -ugauche.serializer -E"test-module 'gauche.serializer" -Eexit
testing bindings in #<module gauche.serializer> ... ERROR: symbols referenced but not defined: self(output-serializer?)
```