DOC: config: fix "Address formats" chapter syntax

The section on "Address formats" doesn't provide the dot (.) after the
chapter numbers, which breaks parsing within the HTML converter.
This commit adds the dot (.) after each chapter within Section 11.

This should be backported to versions 2.4 and above.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 5d62651..c6df770 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -24049,8 +24049,8 @@
 socket type and the transport method.
 
 
-11.1 Address family prefixes
-----------------------------
+11.1. Address family prefixes
+-----------------------------
 
 'abns@<name>' following <name> is an abstract namespace (Linux only).
 
@@ -24085,8 +24085,8 @@
                start by slash '/'.
 
 
-11.2 Socket type prefixes
--------------------------
+11.2. Socket type prefixes
+--------------------------
 
 Previous "Address family prefixes" can also be prefixed to force the socket
 type and the transport method. The default depends of the statement using
@@ -24115,8 +24115,8 @@
 
 
 
-11.3 Protocol prefixes
-----------------------
+11.3. Protocol prefixes
+-----------------------
 
 'quic4@<address>[:port1[-port2]]' following <address> is always considered as
                                   an IPv4 address but socket type is forced to