docs: Fix snapshot name in SET TRANSACTION docs.

Commit 6c2003f8 changed the snapshot names mentioned in
SET TRANSACTION docs, however, there was one place that
the commit missed updating the name.

Back-patch to all supported versions.

Author: Japin Li
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/MEYP282MB1669BD4280044501165F8B07B64F9@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

doc/src/sgml/ref/set_transaction.sgml

@@ -164,7 +164,7 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
    snapshot identifier, which must be given to <literal>SET TRANSACTION
    SNAPSHOT</literal> to specify which snapshot is to be imported.  The
    identifier must be written as a string literal in this command, for example
-   <literal>'000003A1-1'</literal>.
+   <literal>'00000003-0000001B-1'</literal>.
    <literal>SET TRANSACTION SNAPSHOT</literal> can only be executed at the
    start of a transaction, before the first query or
    data-modification statement (<command>SELECT</command>,