Update instructions to set up GDK and Snowplow Micro

What does this MR do and why?

When I was going through the steps in setting up GDK and Snowplow Micro I just copy-pasted the whole command in my terminal not realizing the second line was meant to be run inside psql.

This MR updates the instructions to clarify that the command gdk psql -d gitlabhq_development should be run in the shell while update application_settings set snowplow_collector_hostname='localhost:9090', snowplow_enabled=true, snowplow_cookie_domain='.gitlab.com'; should be run in psql.

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