Clarify Duo SMS MFA instructions

[jreiland]

Per a user: "It might be helpful to add "you will get a permission denied response to the password,sms attempt in the terminal" to the documentation. Just because that is what was throwing me off."

[monaghaa]

Modify step 3 of SMS tab of FAQ:

A list of one-time-passwords (OTPs) will be texted to you via SMS. While you are waiting for the SMS message, you'll see a "Permission denied, please try again" message followed by another login prompt. Now, using the first OTP in the list, login with your password as mypassword,OTP. For example, if my password is Ih3artdu0 and my OTP is 330456 I would type (without quotes): Ih3artdu0,330456.

Example (noting that the password will not appear on your screen like below):

ralphie@login.rc.colorado.edu's password: Ih3artdu0,sms
Permission denied, please try again.
ralphie@login.rc.colorado.edu's password: Ih3artdu0,330456