Doc: fix outdated protocol version.

In the description of StartupMessage, the protocol version was left
3.0. Instead of just updating it, this commit removes the hard coded
protocol version and shows the numbers as an example. This makes that
the part of the doc does not need to be updated when the version is
changed in the future.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/20250626.155608.568829483879866256.ishii%40postgresql.org

doc/src/sgml/protocol.sgml

@@ -6081,13 +6081,14 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
       </varlistentry>
 
       <varlistentry>
-       <term>Int32(196608)</term>
+       <term>Int32</term>
        <listitem>
         <para>
          The protocol version number.  The most significant 16 bits are
-         the major version number (3 for the protocol described here).
-         The least significant 16 bits are the minor version number
-         (0 for the protocol described here).
+         the major version number. The least significant 16 bits are the minor
+         version number. As an example protocol version 3.2 is represented as
+         <literal>196610</literal> in decimal or more clearly as
+         <literal>0x00030002</literal> in hexadecimal.
         </para>
        </listitem>
       </varlistentry>