Fix Dutch translation in membership notification email incorrect

Thijs de Jong
2018-09-26T20:18:35+02:00

Closes #686

#### Previous behaviour
Steps to reproduce:

1. ./manage.py sendmembershipnotification

#### New behaviour
Steps to validate that it works:

1. ./manage.py sendmembershipnotification

Closes #686 (closed)

Previous behaviour

Steps to reproduce:

1. ./manage.py sendmembershipnotification

New behaviour

Steps to validate that it works:

1. ./manage.py sendmembershipnotification