Fix pipe to base64 -d to -D

Mac: `-d` does not work -D does

Eg. echo "c3R...zE2" | base64 -D

Author: ch4nd4n

content/en/docs/tasks/configure-pod-container/pull-image-private-registry.md

@@ -90,15 +90,15 @@ The value of the `.dockerconfigjson` field is a base64 representation of your Do
 To understand what is in the `.dockerconfigjson` field, convert the secret data to a
 readable format:
 
-    kubectl get secret regcred --output="jsonpath={.data.\.dockerconfigjson}" | base64 -d
+    kubectl get secret regcred --output="jsonpath={.data.\.dockerconfigjson}" | base64 -D
 
 The output is similar to this:
 
     {"auths":{"yourprivateregistry.com":{"username":"janedoe","password":"xxxxxxxxxxx","email":"[email protected]","auth":"c3R...zE2"}}}
 
 To understand what is in the `auth` field, convert the base64-encoded data to a readable format:
 
-    echo "c3R...zE2" | base64 -d
+    echo "c3R...zE2" | base64 -D
 
 The output, username and password concatenated with a `:`, is similar to this: