Fix event trigger example

Fix event trigger example

Commit 2f9661311b changed command tags from strings to numbers, but
forgot to adjust the code in the event trigger example, which
consequently failed to compile.

While fixing that, improve the indentation to adhere to pgindent style.

Backpatch to v13, where the change was introduced.

Author: Laurenz Albe
Discussion: https://postgr.es/m/81e36ac17dc80489e74dc5b6914afa6ccdb1a99d[email protected]