Fix error in README.md (Direct)

[sdesalas]

Following instructions for 'Direct' use in Readme.md creates a TypeError: generate is not a function:

To duplicate follow these steps:

$ node -v && node
v6.10.0
> const generator = require('audio-generator');
undefined
> const generate = generator(time => Math.sin(time * Math.PI * 2 * 440));
undefined
> typeof generate
'object'
> generate()
TypeError: generate is not a function
    at repl:1:1
    at sigintHandlersWrap (vm.js:22:35)
    at sigintHandlersWrap (vm.js:96:12)
    at ContextifyScript.Script.runInThisContext (vm.js:21:12)
    at REPLServer.defaultEval (repl.js:340:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:538:10)
    at emitOne (events.js:101:20)
    at REPLServer.emit (events.js:188:7)

This is happening because package.json is pointing main to stream.js.

Instead of repointing the package.json (which may have undesired effects elsewhere) the workaround here is to point directly to the direct direct.js (Generator) function.