plugin: Fix documentation typo for type_mask

plugin: Fix documentation typo for type_mask

The v3 plug-in API was slightly misleading, as that interface does not
need to use an int pointer to setting the type_mask from the plug-in.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20170508141939.31224-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14576.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 26e3427cfa128c5d8ac7e212769ba29afac4f3d9)