Change modernizr script reference in How To Use

Neither download or repo include a file named Modernizr-yesnope.js but do include modernizr-custom.js.

If the intent of the How To is to suggest devs may use their own version of Modernizr then this should be more explicitly called out as in my second example.

In either case I'm not at all clear why Modernizr-yesnope.js is used, if I create my own production Modernizr from their build tool I don't get that file.
Merge pull request #196 from billsaysthis/patch-1

Change modernizr script reference in How To Use