Update get-started.md to replace next step dead link
Update get-started.md to replace next step dead link

There is currently a dead link in the next steps section of the Getting Started docs that navigates to a URL that is no longer valid. Proposing to change it to a direct link to the Apollo Client API docs so the reader can still access relevant information.
apollostack/apollo-client,apollographql/apollo-client,apollographql/apollo-client,apollostack/apollo-client,apollostack/apollo-client