Update the README.md

The readme has been brought up to date with the govuk styleguide at
https://github.com/alphagov/styleguides/blob/master/use-of-READMEs.md\

https://www.pivotaltracker.com/story/show/84366924
I think referencing preview is confusing here: that startup command is likely to be used on the dev VM, so instead:

  "The app runs on port 3073.  If you're using the dev VM it will be available at http://search-admin.dev.gov.uk/"