Set python version to 3.9 in spark examples (#1725)

Signed-off-by: Eduardo Apolinario <eapolinario@users.noreply.github.com>
Co-authored-by: Eduardo Apolinario <eapolinario@users.noreply.github.com>

Author: eapolinario

examples/k8s_spark_plugin/k8s_spark_plugin/dataframe_passing.py

@@ -19,7 +19,7 @@
 # %% [markdown]
 # Create an `ImageSpec` to automate the retrieval of a prebuilt Spark image.
 # %%
-custom_image = ImageSpec(registry="ghcr.io/flyteorg", packages=["flytekitplugins-spark"])
+custom_image = ImageSpec(python_version="3.9", registry="ghcr.io/flyteorg", packages=["flytekitplugins-spark"])
 
 # %% [markdown]
 # :::{important}

examples/k8s_spark_plugin/k8s_spark_plugin/pyspark_pi.py

@@ -16,7 +16,7 @@
 # %% [markdown]
 # Create an `ImageSpec` to automate the retrieval of a prebuilt Spark image.
 # %%
-custom_image = ImageSpec(registry="ghcr.io/flyteorg", packages=["flytekitplugins-spark"])
+custom_image = ImageSpec(python_version="3.9", registry="ghcr.io/flyteorg", packages=["flytekitplugins-spark"])
 
 
 # %% [markdown]