Wrong and incomplete info about force SSL enable for master node at the docs

[Alexander Kurenyshev]

Here [1] we can find info about forcing enable SSL access for master node.

At the "Additional information" there is a code block:
SSL:
force_https: true

which should contain the following:

"SSL":
  "force_https": :true"

Also there is no info that we need rebuild nginx container after that change.
See [2] commit message for clarification.

[1] https://docs.mirantis.com/fuel/fuel-master/operations.html#switching-on-ssl-and-secure-access
[2] https://review.openstack.org/#/c/210006/