Update docs on how to run the playground

Fixes #6

Author: pimterry

README.md

@@ -38,6 +38,6 @@ To get started:
 
 A few tips:
 
-* Setting `playground: true` at the bottom of [`src/httptoolkit-server.ts`](src/httptoolkit-server.ts) will give you a GraphQL playground on http://localhost:45457.
+* Setting `playground: '/'` at the bottom of [`src/httptoolkit-server.ts`](src/httptoolkit-server.ts) will give you a GraphQL playground on http://localhost:45457/.
 * New interceptors need to be added to `src/interceptors/index.ts`. They will also need to be added to [the UI](https://github.com/httptoolkit/httptoolkit-ui).
 * The tests assume that the required applications are installed to run every interceptor. See [.travis.yml](.travis.yml) for an example of how to set this up.