A couple of corrections to lesson 4

A few minor corrections for readability.
Update tutorial.md
[ruby] Add 'define' to the explanation of methods
[ruby] Few changes to instance variables, lesson 4

It seems a bit strange to start using instance variables in the section before they're introduced. Replacing them with `puts` calls so that folks can **see** the arguments being passed in.
Merge pull request #65 from BRMatt/patch-1

A couple of corrections to lesson 4