fix: Change incorrect reference to object key in newsletter signup function

civicrm.forms describes a subscription form and defines its fields, which include email_address. The method which received subscription intent erroneously referred to this field with the key email, which caused an error to be thrown. This commit amends this reference to the proper email_address key, and this method should now operate as intended. Unit testing surrounding these methods is intended to follow shortly, but a fix for this process takes priority in order to unblock testing.