Cannot compile example macOS application

[sidepelican]

Checklist

• This is not a Apple's bug.
• Reviewed the README and documents.
• Searched existing issues for ensure not duplicated.

Expected Behavior

We can compile example macOS application

Current Behavior

has compile error.

DiffableDataSources/Examples/Example-macOS/MountainsViewController.swift

Line 52 in 581b4f8

let snapshot = DiffableDataSourceSnapshot<Section, Mountain>()

this let should be var

Steps to Reproduce

1. just run Example-macOS

Environments

• version: 0.4.0

• Swift version: 5.2.4

• Xcode version: 11.6