Update python syntax in OAuth2 example

Author: headcr4sh

docs/examples/auth/oauth-external-auth/README.md

@@ -55,7 +55,7 @@ kubectl create -f https://raw.githubusercontent.com/kubernetes/kops/master/addon
 
 - OAUTH2_PROXY_CLIENT_ID with the github `<Client ID>`
 - OAUTH2_PROXY_CLIENT_SECRET with the github `<Client Secret>`
-- OAUTH2_PROXY_COOKIE_SECRET with value of `python -c 'import os,base64; print base64.b64encode(os.urandom(16))'`
+- OAUTH2_PROXY_COOKIE_SECRET with value of `python -c 'import os,base64; print(base64.b64encode(os.urandom(16)).decode("ascii"))'`
 
 4. Customize the contents of the file dashboard-ingress.yaml: