Update a prompt text in DataPreview

[ykiu]

What this PR does

The functionality of adding multiple datasets at once by pressing ctrl/shift is great, but is hard to discover.

This commit improves the message displayed in the DataPreview to make the feature more discoverable.

Test me

Launch TerriaMap and see if the message is appropriate.

Checklist

• [ ] There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
  • unit tests are not applicable because there're no changes to the code
• [ ] I've updated relevant documentation in doc/.
• [x] I've updated CHANGES.md with what I changed.
• [x] I've provided instructions in the PR description on how to test this PR.