Update install instructions in docs (#318)

Author: rossjones

lib/importer/assets/docs/start.html

@@ -128,7 +128,7 @@ <h2 class="govuk-heading-l">How to install it</h2>
               <p>
                   Open up your preferred command line, which you previously used to create a GOV.UK Prototype, and type the following:
               </p>
-              <pre>npm install 'https://gitpkg.vercel.app/register-dynamics/data-import/lib/importer?main'</pre>
+              <pre>npm install --save  @register-dynamics/importer@latest</pre>
               <p>
                 After running this command new routes will automatically be added
                 to your prototype’s <code>app/routes.js</code> file.