`ui_material` example should show how to use `Globals`

## How can Bevy's documentation be improved?

It's easy for a user to get tripped up by using the wrong globals import, e.g. from `bevy_pbr`. We don't seem to show the correct import anywhere in our docs or examples.

Something like
```c
#import bevy_render::globals::Globals

@group(0) @binding(1) var globals: Globals;

// globals.time
```
Seems to work, but I'm not 100% sure if this is the canonical way to do it.