[Bug]: Fix `get_bounds()` error message

[tomvothecoder]

What happened?

The error message for get_bounds() incorrect says "their related time coordinate variables".

https://github.com/xCDAT/xcdat/blob/e3dda64feab046046b6a3ee8047faec1ab01f8ec/xcdat/bounds.py#L256

What did you expect to happen? Are there are possible answers you came across?

It should just be "their related coordinate variables"

Minimal Complete Verifiable Example (MVCE)

No response

Relevant log output

No response

Anything else we need to know?

No response

Environment

main and latest xCDAT