Fixed to to match current setup instructions (#397)

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

Author: gunnim

Diff for: 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
 					);

Diff for: 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;

Diff for: 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().");
 			}
 		}