Update getting_started.md

I changed a tip below some ruby code in "5.7 showing posts"
to make it apparent that anything below the keyword 'private' is also included as being private rather then just the definition immediately below it

it now reads as :

Note that `def post_params` is private, this also includes everything below the keyword 'private'. 
This new approach prevents an attacker from setting the model's attributes by manipulating the hash 
passed to the model. If you encounter an error about missing params try moving the whole function above
the 'private' keyword.