Add "io worker" as backend_type to the pg_stat_activity documentation

I happened to notice that "io worker" was missing in the list of
backend types in the pg_stat_activity documentation.

Here is a patch to fix that.

I wondered vaguely if the list should be sorted alphabetically,
but didn't do it a) to keep the patch easy to read and
b) because the list is not long enough for readers to get lost.

Yours,
Laurenz Albe