Prefer 'standardisation' to 'normalisation' in image section

[ots22]

• This makes it consistent with definitions given earlier
• Some code cells still need to be updated
• The example standardises then normalises - perhaps could add a little more explanation

[lannelin]

@ots22 thanks for catching this and raising the PR!

I've made a couple of changes to the code cells for consistency.

I've also added a very brief, general, note for normalisation vs standardisation in the image context.

If you're happy with the new note, then I'll merge this in tomorrow morning pre-teaching.

[ots22]

LGTM!