`nginx_proxy` role typo

I came across an error when running helper script seed-playbook-dev:

ERROR! the role 'nginx_proxy' was not found in /home/eric/code/mfmt/seed/roles:/home/eric/code/mfmt/seed/roles:/home/eric/code/mfmt/seed

The error appears to be in '/home/eric/code/mfmt/seed/site-dev.yml': line 126, column 7, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

  roles:
    - nginx_proxy
      ^ here

Based on the error message, it looks like a typo. It should be nginx-proxy instead of nginx_proxy. I'll send a PR.