pgsql: doc: Add link to login event trigger example

doc: Add link to login event trigger example

The login event trigger is not listed on the trigger firing matrix
since it's not fired by a command.  Add a link to the example code
page similar to how the other event triggers link to the matrix.

Reported-by: Marcos Pegoraro <marcos@f10.com.br>
Discussion: https://postgr.es/m/CAB-JLwYS+78rX02BZ3wJ9ykVrd2i3O1K+7jzvZKQ0evquyQiLQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/94902b146fae024c03f80d496a0da5ba9751c9b5

Modified Files
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doc/src/sgml/event-trigger.sgml | 5 +++++
1 file changed, 5 insertions(+)