documentation: "mixin" instead of "mixins"

[unoriginalscreenname]

Hey, i noticed that you're using "mixin" in your example instead of "mixins"

return flux.store({
        mixin: [MyMixin],
        todos: [],

It took me a minute to figure out why the mixins weren't working. You need to change that to "mixins".

[christianalfoni]

Hi @ericmcgregor and thanks so much for pointing that out, it has been fixed!

[unoriginalscreenname]

Thanks! also, great job on the library!