Add missing comma to GlobalHook.js injected script

The perils of creating JS in a string... :-)
Have you considered doing something like:
```
var js = (function() {
Object.defineProperty(
    window, '__REACT_DEVTOOLS_GLOBAL_HOOK__', {value: {
        inject: function(runtime) { this._reactRuntime = runtime; },
        getSelectedInstance: null,
        Overlay: null
    }}
);
}).toString() + "()";
```

a bit sketchy but it's lintable. I can change my pull request for something like this if you're happy with the suggestion.