MONGOID-5518: Add Puma dependency to Sinatra tutorial (#5516)

Author: alexbevi

docs/tutorials/getting-started-sinatra.txt

@@ -44,8 +44,9 @@ Create a file named ``Gemfile`` with the following contents:
 
     source 'https://rubygems.org'
 
-    gem 'sinatra', '~> 2.0'
-    gem 'mongoid', '~> 7.0'
+    gem 'sinatra'
+    gem 'mongoid'
+    gem 'puma'
 
 
 Install Dependencies