docs: Fix protocol version 3.2 message format of CancelRequest

Since protocol version 3.2 the CancelRequest does not have a fixed size
length anymore. The protocol docs still listed the length field to be a
constant number though. This fixes that.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reported-by: Dmitry Igrishin <dmitigr@gmail.com>
Backpatch-through: 18

doc/src/sgml/protocol.sgml

@@ -4354,7 +4354,7 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
     <listitem>
      <variablelist>
       <varlistentry>
-       <term>Int32(16)</term>
+       <term>Int32</term>
        <listitem>
         <para>
          Length of message contents in bytes, including self.