man: add links to directive index to see-alsos

systemd.directives(5) is renamed to systemd.directives(7).
Section 7 is "Miscellaneous".
diff --git a/make-directive-index.py b/make-directive-index.py
index e94f4c5..48d830e 100755
--- a/make-directive-index.py
+++ b/make-directive-index.py
@@ -40,7 +40,7 @@
 
         <refmeta>
                 <refentrytitle>systemd.directives</refentrytitle>
-                <manvolnum>5</manvolnum>
+                <manvolnum>7</manvolnum>
         </refmeta>
 
         <refnamediv>