[services] Adjust tabs to be two spaces in build_base_image.sh

Summary: Minor nit, I left the comment after my accept in D3234 so it was probably missed.

Test Plan: N/A

Reviewers: jimpo

Reviewed By: jimpo

Subscribers: palys-swm, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3395