sinatra-app-tutorial: Install webrick

New versions of Sinatra require you to install a server.
webrick is a simple server with two main benefits when it comes to
conducting a Rails Girls workshop:

1) It runs in a single-threaded mode by default, unlike puma. This ensures
   the attendees won't ever run observe some weird side effects of request 2
   returning before request 1.
2) It doesn't need any native extensions which reduces build time and
   the chance of running into an errors during installation.

_pages/sinatra-app-tutorial.md

@@ -22,7 +22,7 @@ Explain shortly what [Sinatra](https://sinatrarb.com) is.
 
 Remember how we needed to install Ruby on Rails? Similarly we need to install Sinatra:
 
-`gem install sinatra`
+`gem install sinatra webrick`
 
 ### Create your first Sinatra app