Update README example

The commit eb36f56 adds a default value to base_url for class PodmanClient, I'm not sure whether the docs strings in the class itself should also specify that base_url now has a default value, but in any case the example in README.md can be made simpler like this:

```
"""Demonstrate PodmanClient."""
import json
from podman import PodmanClient

with PodmanClient() as client:
version = client.version()
print("Release: ", version["Version"])
print("Compatible API: ", version["ApiVersion"])
print("Podman API: ", version["Components"][0]["Details"]["APIVersion"], "\n")

# get all images
for image in client.images.list():
print(image, image.id, "\n")

# find all containers
for container in client.containers.list():
# After a list call you would probably want to reload the container
# to get the information about the variables such as status.
# Note that list() ignores the sparse option and assumes True by default.
container.reload()
print(container, container.id, "\n")
print(container, container.status, "\n")

# available fields
print(sorted(container.attrs.keys()))

print(json.dumps(client.df(), indent=4))
```