Update elasticsearch installation docs

Why is this change being made?

While setting up the GDK with elasticsearch support, I've noticed that altering gdk.yml and running gdk reconfigure is not enough to run an elasticsearch instance. It started working only after I uncommented elasticsearch service in Procfile.

This MR fixes the installation steps in the docs.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what