doc: Fix link to SELinux user guide in sepgsql page

Reported-by: Anton Voloshin
Discussion: https://postgr.es/m/15a86d4e-a237-1acd-18a2-fd69730f1ab9@postgrespro.ru
Backpatch-through: 10

Author: michaelpq

doc/src/sgml/sepgsql.sgml

@@ -797,7 +797,7 @@ ERROR:  SELinux: security policy violation
     </listitem>
    </varlistentry>
    <varlistentry>
-    <term><ulink url="https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/SELinux_Users_and_Administrators_Guide/">SELinux User's and Administrator's Guide</ulink></term>
+    <term><ulink url="https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/selinux_users_and_administrators_guide/index">SELinux User's and Administrator's Guide</ulink></term>
     <listitem>
      <para>
       This document provides a wide spectrum of knowledge to administer