Fix descriptions of the default API version in docs

[thombashi]

The default value of the version of APIClient described as 1.30 in the docker-py document.
However, it is actually 1.35.
self._version assigned DEFAULT_DOCKER_API_VERSION if passing None to version, and its value is 1.35

docker-py/docker/constants.py

Line 4 in e1e4048

DEFAULT_DOCKER_API_VERSION = '1.35'

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "fix_api_ver_desc" [email protected]:thombashi/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[shin-]

Thanks!