Fix example for MandrillHelper.put_param/3

[shawnleong]

Thanks for open-sourcing and the work behind bamboo. I was perusing the docs and got slightly confused by the example before realising it was an error in the given example.

Background

The example for Bamboo.MandrillHelper.put_param/3 is incorrectly calling on a non-existent put_param/4

• The example passes email twice to the function
• First through the pipeline
• Second through the function argument

Fix

This merge request aims to corrects the example in the documentation.