README.md file in when-are-pubsub-messages-acked

[XiaoxiaLin]

a typo in the README.md

should change "input-topic" to "input-sub" in the screenshot below:

$ gcloud pubsub subscriptions pull input-sub --limit 10 --auto-ack

[XiaoxiaLin]

some typo when pulling msg from input-sub after publish message "test".

[gxercavins]

Thanks, corrected now