virsh: Add source-protocol-ver to pool-define-as docs

Commit a3dbaa36 neglected to add the source-protocol-ver to the
pool-define-as command.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
ACKed-by: NMichal Privoznik <mprivozn@redhat.com>

tools/virsh.pod

@@ -4013,6 +4013,7 @@ from the XML I<file>.
 [I<--source-name name>] [I<--target path>] [I<--source-format format>]
 [I<--auth-type authtype> I<--auth-username username>
 [I<--secret-usage usage> | I<--secret-uuid uuid>]]
+[I<--source-protocol-ver ver>]
 [[I<--adapter-name name>] | [I<--adapter-wwnn> I<--adapter-wwpn>]
 [I<--adapter-parent parent>]] [I<--print-xml>]