doc: Fix stand-alone INSTALL file build

Commit 7d17e683 introduced an external
link.

doc/src/sgml/installation.sgml

@@ -820,8 +820,8 @@ su - postgres
          for <application>systemd</application><indexterm><primary>systemd</primary></indexterm>
          service notifications.  This improves integration if the server binary
          is started under <application>systemd</application> but has no impact
-         otherwise; see <xref linkend="server-start"> for more
-         information.  <application>libsystemd</application> and the
+         otherwise<![%standalone-ignore[; see <xref linkend="server-start"> for more
+         information]]>.  <application>libsystemd</application> and the
          associated header files need to be installed to be able to use this
          option.
         </para>