Update problem.md to include link to Generator documentation

I've added a link to Mozilla's documentation on Generators - It took me a while to realize that function * is a special syntax and "*" is not meant to be just a placeholder in case developer wants to give the function a name ... 
I wasn't aware of the Generator proposal ... perhaps others aren't aware what it is either
Because I didnt see it when I copied it ... sorry was not meant to be there