Add comma before callback for code example
Add comma before callback for code example

The example code for the following functions had a missing comma to 
separate the callback function to the rest of the parameters:
- webAuth.redirect.loginWithCredentials
- webAuth.popup.loginWithCredentials
- webAuth.redirect.login
auth0/docs,jeffreylees/docs,auth0/docs,yvonnewilson/docs,auth0/docs,jeffreylees/docs,yvonnewilson/docs,jeffreylees/docs,yvonnewilson/docs