add-subnet help text incorrect

[james beedy]

`juju help add-subnet` gives some info pertaining to the `create-subnet` command which doesn't exist anymore.

[james beedy (jamesbeedy)]

ubuntu@ip-10-20-100-90:~$ juju help add-subnet
Usage: juju add-subnet [options] <CIDR>|<provider-id> <space> [<zone1> <zone2> ...]

Summary:
Add an existing subnet to Juju.

Options:
-B, --no-browser-login (= false)
    Do not use web browser for authentication
-m, --model (= "")
    Model to operate in. Accepts [<controller name>:]<model name>

Details:
Adds an existing subnet to Juju, making it available for use. Unlike
"juju create-subnet", this command does not create a new subnet, so it
is supported on a wider variety of clouds (where SDN features are not
available, e.g. MAAS). The subnet will be associated with the given
existing Juju network space.

Subnets can be referenced by either their CIDR or ProviderId (if the
provider supports it). If CIDR is used an multiple subnets have the
same CIDR, an error will be returned, including the list of possible
provider IDs uniquely identifying each subnet.

Any availablility zones associated with the added subnet are automatically
discovered using the cloud API (if supported). If this is not possible,
since any subnet needs to be part of at least one zone, specifying
zone(s) is required.

[Reed O'Brien (reedobrien)]

PR: https://github.com/juju/juju/pull/6938