fix documentation of hideAfter configuration option in README

[m-idler]

Other than currently documented, hideAfter is used from inside the layout property instead of directly from within config.

usage:

MMM-BluetoothDevices/MMM-BluetoothDevices.js

Line 132 in ffef40c

const hideAfter = this.config.layout.hideAfter || this.defaults.layout.hideAfter;