Fix "how it works" readme example

[tadeucaldas]

In your readme, in the "Hot it Works" section, says to use

..... new BroadcastStrategy({key:.....})........

if u do that the console shows:

Uncaught TypeError: vuex_shared_mutations__WEBPACK_IMPORTED_MODULE_2__.BroadcastStrategy is not a constructor

What u really must do is

..... new BroadcastChannelStrategy({key:.....})........

The Channel part is missing... at least that's how i made it work