[motion-1] Invalid offset-path value syntax

The `ray()` and `path()` functions in the `offset-path` value definition must be wrapped in `<>` to identify them as CSS types.

https://github.com/w3c/fxtf-drafts/blob/cb4c0bb766ccc4a6670c240107c934865389089c/motion-1/Overview.bs#L106

The correct syntax should be:
`none | <ray()> | <path()> | <url> | [ <basic-shape> && <coord-box>? ] | <coord-box>`