doc: restore intentional typo

Commit ac8623760 "fixed" a typo in an example of what would happen in
the event of a typo. Restore the original typo and add a comment about
its intentionality. Backpatch to 12 where the error was introduced.

Per report from irc user Nicolás Alvarez.

doc/src/sgml/protocol.sgml

@@ -811,7 +811,7 @@ BEGIN;
 INSERT INTO mytable VALUES(1);
 COMMIT;
 INSERT INTO mytable VALUES(2);
-SELECT 1/0;
+SELCT 1/0;<!-- this typo is intentional -->
 </programlisting>
      then none of the statements would get run, resulting in the visible
      difference that the first <command>INSERT</command> is not committed.