nixos/release-notes: fix `slaptest` command for openldap section

When running - as suggested - `slaptest -f slapd.conf $TMPDIR` I get the
following result:

    [root@ldap:/tmp/tmp.De46ABIbFf]# slaptest -f /nix/store/lks3ihydj40ff6yqvz0k33ycrc9vbyry-slapd.conf $TMPDIR
    usage: slaptest [-v] [-d debuglevel] [-f configfile] [-F configdir] [-o <name>[=<value>]] [-n databasenumber] [-u] [-Q]

    [root@ldap:/tmp/tmp.De46ABIbFf]# echo $?
    1

Adding a `-F` option fixes the issue.

(cherry picked from commit b5a12b4b61)

nixos/doc/manual/release-notes/rl-2105.xml

@@ -427,7 +427,7 @@
     </para>
     <programlisting>
       TMPDIR=$(mktemp -d)
-      slaptest -f /path/to/slapd.conf $TMPDIR
+      slaptest -f /path/to/slapd.conf -F $TMPDIR
       slapcat -F $TMPDIR -n0 -H 'ldap:///???(!(objectClass=olcSchemaConfig))'
     </programlisting>
     <para>