This usually happens when you miss accepting the RSA Key Notification at first.
Best would be to reset the setting.
Do as follows:
- Switch OFF - Developer's option.
 
- Under Developer Setting, Tap on - Revoke USB Authorization 
 
- Switch ON Developer's option 
- Enable USB Debugging, and you would see RSA Key Notification on device
  
 - You should immediately see the above notifiaction. 
- Select - Always allow from this computerand Ok.
 To avoid further denial.
 
The link mentioned in your question should also work.
Might be some issue.
Update:
This might also be an Issue with your adb binary version.
Check adb version using:
$ adb version
Android Debug Bridge version 1.0.31
If you are using an older version please update.