Add the modprobe.d name to allow man(1) to find both pages.

--- a/doc/modprobe.conf.sgml
+++ b/doc/modprobe.conf.sgml
@@ -23,6 +23,8 @@
   <refmeta>
     <refentrytitle>modprobe.conf</refentrytitle>
     <manvolnum>5</manvolnum>
+    <refentrytitle>modprobe.d</refentrytitle>
+    <manvolnum>5</manvolnum>
   </refmeta>
   <refnamediv>
     <refname>modprobe.d</refname><refname>modprobe.conf</refname> <refpurpose>Configuration directory/file for modprobe</refpurpose>