[AIRFLOW-2869] Remove smart quote from default config

Closes apache#3716 from wdhorton/remove-smart-quote-
from-cfg

(cherry picked from commit 67e2bb9)
Signed-off-by: Bolke de Bruin <bolke@xs4all.nl>

Author: wdhorton

airflow/config_templates/default_airflow.cfg

@@ -606,7 +606,7 @@ image_pull_secrets =
 gcp_service_account_keys =
 
 # Use the service account kubernetes gives to pods to connect to kubernetes cluster.
-# It’s intended for clients that expect to be running inside a pod running on kubernetes.
+# It's intended for clients that expect to be running inside a pod running on kubernetes.
 # It will raise an exception if called from a process not running in a kubernetes environment.
 in_cluster = True