Update pythonVersion to fix example

[stratus]

spark 3.1.1 (specified in image) doesn't work with Python 2.
This change updates pythonVersion from 2 to 3.

[stratus]

FWIW, I've tested this on a Mac as follows (w/o the sed call I'd get the python version related error):

kind create cluster --name spark-foo
kubectl cluster-info --context kind-spark-foo

git clone https://github.com/GoogleCloudPlatform/spark-on-k8s-operator
cd spark-on-k8s-operator
sed -i '' 's/pythonVersion: "2"/pythonVersion: "3"/' examples/spark-py-pi.yaml

helm repo add spark-operator https://googlecloudplatform.github.io/spark-on-k8s-operator
helm install my-release spark-operator/spark-operator
--namespace spark-operator --create-namespace
--set webhook.enable=true --set image.tag=latest

kubectl apply -f manifest/spark-rbac.yaml
kubectl apply -f examples/spark-py-pi.yaml