iPad getting annoyed by upgrade requests
As mentioned in the comments, your iPad (MC764LL/A) is an iPad 2, and not the original iPad as mentioned. 
The latest version of iOS on your iPad is iOS 9.3.2. Since you are using iOS 9.1, you will get an update notification. You could, however, choose to ignore it. 
I would suggest you install the new OS. 
To answer your question: No, you cannot turn off the notification for an iOS software update.