Fix h1 heading in ruby tutorial

GitHub doesn't seem to display the second `#` / h1 heading.

Author: tijmenb

ruby/lesson3/tutorial.md

@@ -49,7 +49,7 @@ end
 > Don't forget to wrap your interactions within `transaction` blocks
 
 
-# Exercise: Contacts list
+## Exercise: Contacts list
 
 Using what we have learned in the last couple of ruby lessons we will create a contact list where we can store people's names and date of birth.