Fixed to to match current setup instructions (#397)

According to
https://reactjs.net/getting-started/tutorial.html
you run services.AddReact() not app.AddReact()

src/React.AspNet/HtmlHelperExtensions.cs

@@ -50,7 +50,7 @@ private static IReactEnvironment Environment
  "ReactJS.NET has not been initialised correctly.",
 #else
  "ReactJS.NET has not been initialised correctly. Please ensure you have " +
- "called app.AddReact() and app.UseReact() in your Startup.cs file.",
+ "called services.AddReact() and app.UseReact() in your Startup.cs file.",
 #endif
  ex
  );

src/React.AspNet/HttpContextLifetimeProvider.cs

@@ -63,7 +63,7 @@ private PerRequestRegistrations Registrations
  {
  throw new ReactNotInitialisedException(
  "ReactJS.NET has not been initialised correctly. Please ensure you have " +
- "called app.AddReact() and app.UseReact() in your Startup.cs file."
+ "called services.AddReact() and app.UseReact() in your Startup.cs file."
  );
  }
  return registrations;

src/React.AspNet/ReactBuilderExtensions.cs

@@ -88,7 +88,7 @@ private static void EnsureServicesRegistered(IApplicationBuilder app)
  var registrations = app.ApplicationServices.GetService<HttpContextLifetimeProvider.PerRequestRegistrations>();
  if (registrations == null)
  {
- throw new ReactNotInitialisedException("Please call app.AddReact() before app.UseReact().");
+ throw new ReactNotInitialisedException("Please call services.AddReact() before app.UseReact().");
  }
  }