Change extra RAM/CPU docs in default Vagrantfile
Change extra RAM/CPU docs in default Vagrantfile

The default RAM was changed in 79e4ecf, so the documentation no longer
made sense as it was.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet