Update documentation for the default value of main set worker-threads

Update documentation for the default value of main set worker-threads

I forgot to update this documentation when I changed the default for
maint set worker-threads to be 0. This is a branch-only change, because
on trunk this has been changed back to unlimited.

gdb/doc/ChangeLog:

2019-12-15  Christian Biesinger  <cbiesinger@google.com>

	* gdb.texinfo (Maintenance Commands): Update documentation for
	maint set worker-threads to say the default is 0.

Change-Id: I75b74c5da599e657ee94daa8c88b5a097bc58d15