[ci skip] Consistency wording of 9.6 in form_helpers.md

Chapter 9 is giving a user and set of addresses example.
So 'Add a new address' would be more consistent.
Also change the javascript to JavaScript.

guides/source/form_helpers.md

@@ -973,4 +973,4 @@ As a convenience you can instead pass the symbol `:all_blank` which will create
 
 ### Adding Fields on the Fly
 
-Rather than rendering multiple sets of fields ahead of time you may wish to add them only when a user clicks on an 'Add new child' button. Rails does not provide any builtin support for this. When generating new sets of fields you must ensure the key of the associated array is unique - the current javascript date (milliseconds after the epoch) is a common choice.
+Rather than rendering multiple sets of fields ahead of time you may wish to add them only when a user clicks on an 'Add new address' button. Rails does not provide any builtin support for this. When generating new sets of fields you must ensure the key of the associated array is unique - the current JavaScript date (milliseconds after the epoch) is a common choice.