fix: adds key attribute to help react find dom changes on generated div

[DeveloperMetal]

When you create elements in a loop like the users.map(...) in your code you need to tell react how to target these divs when an update happens.

React doesn't know how or which of these generated divs should be updated if the users state changes. So to help it you need to include the key attribute and pass it a unique id. It can be anything, but must be unique to this loop.

See here for more info: https://reactjs.org/docs/lists-and-keys.html