make migration in example docker setup more robust

[pniederlag]

In migration of the docker example there is a step included that executes a sql query (psql -c \"CREATE pg_trgm;") this step will only work on a pristine database. Any subsequent executions will result in an error.

I updated the query to be more robust so I can reexecute/force the migration "setup" by modifying the query to:

psql -c \"CREATE EXTENSION IF NOT EXISTS pg_trgm;

https://github.com/camptocamp/docker-odoo-project/blob/8f7a8608063bb5a879b8c5172ddd71fd9679094f/example/odoo/migration.yml#L8