Revert "Update example with sub.subscribe('')"

This reverts commit ce6338b5d1ae61c102cefc381760cd2e525abecc.
SubSocket#subscribe is no longer in the API; instead,
SubSocket#connect accepts another argument.
Give topic a default value if not supplied

It's possible to use SubSocket#connect without a callback *or* topic,
in which case it ought to use an empty string for the topic.