Correct copy/paste mistake in the DECLARE_INSTANCE_CHECKER and
DECLARE_CLASS_CHECKERS documentation.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201003025424.199291-2-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>