Remove Subscriptions API version from top-level description of the SDK Subscription client

[JuLeeAtPlanet]

There is no reason to call out the version of the Subscriptions client.

• Orders & Data do not call out those "versions"
• we're not really doing proper versioning at this point anyway
• might confuse folks down the road, as this is the V2 SDK

Let's change text from:

SubscriptionsClient ¶
A Planet Subscriptions Service API 1.0.0 client.
The methods of this class forward request parameters to the operations described in the Planet Subscriptions Service API 1.0.0 (https://api.planet.com/subscriptions/v1/spec) using HTTP 1.1.

And conform it to the other APIs with the new text:

High-level asynchronous access to Planet's Subscription API. The methods of this class forward request parameters to the operations described in the Planet Subscriptions Service API (https://api.planet.com/subscriptions/v1/spec).

[JuLeeAtPlanet]

Current published docs have correct versioning.