env_var: Fix POST parameter name
env_var: Fix POST parameter name

In the `create` method the POST parameter for the environment variable name is currently incorrect. It's changed to the value described in the API documentation: `name`.

Change-type: patch
Signed-off-by: Trong Nghia Nguyen <8e1f45cdcce8e4aef30b6ef5f5ba85d3ec052bb0@gmail.com>

resin-io/resin-sdk-python,resin-io/resin-sdk-python