Setup Chapter in Book Has Unclear Event Struct

## Description

In chapter 7 of the book, under [custom setup functionality](https://specs.amethyst.rs/docs/tutorials/07_setup#custom-setup-functionality), the tutorial instructs the user to create a struct `Sys` with a field `reader` that looks like `ReaderId`. `cargo check` gives the error ```cannot find type `Event` in this scope```. If I try to import `specs::shrev::Event`, that doesn't really work since Event is a trait.

## Meta

Rust version: 1.47.0
Specs version / commit: 0.16.1
Operating system: macOS 10.15.6

## Reproduction

Steps to reproduce the behavior:
1. Go to [setup](https://specs.amethyst.rs/docs/tutorials/07_setup)
2. Go through the tutorial up to Custom Setup Functionality
3. Run `cargo check` after doing the first code snippet in Custom Setup Functionality

## Expected behavior

The code returns no errors or warnings. I am willing to submit a PR over this issue, but I am unsure what Event should be