Make example in README runnable. (#113)

README.md

@@ -42,9 +42,10 @@ To support all of DigitalOcean's HTTP APIs, a generated library is available whi
 Find below a working example for GET a ssh_key ([per this http request](https://docs.digitalocean.com/reference/api/api-reference/#operation/sshKeys_list)) and printing the ID associated with the ssh key. If you'd like to try out this quick example, you can follow [these instructions](https://docs.digitalocean.com/products/droplets/how-to/add-ssh-keys/) to add ssh keys to your DO account.
 
 ```python
+import os
 from pydo import Client
 
-client = Client(token=$DIGITALOCEAN_TOKEN)
+client = Client(token=os.getenv("$DIGITALOCEAN_TOKEN"))
 
 ssh_keys_resp = client.ssh_keys.list()
 for k in ssh_keys_resp["ssh_keys"]: