_atom_type.display_colour typo fix

[rowlesmr]

Changed colour_rgb to colour_RGB.

Everything else uses uppercase RGB.

[jamesrhester]

This is perhaps unexpected, but our dictionary style guide points 4.3.1 and 5.1 explicitly specify all lowercase for save frame headings, and therefore by extension references to save frame headings.

Interestingly, all checks passed indicating that these particular rules are not checked by the style checker. I should fix that.

If you think this style could be improved, please raise an issue in the comcifs.github.io repository.

[jamesrhester]

And yes, I realise that our imported save frames don't yet conform to the style rules. I'll add it to the list of issues.

[rowlesmr]

so (for example) save_pd_pref_orient_March_Dollase.diffractogram_block_id should be save_pd_pref_orient_march_dollase.diffractogram_block_id?

brb changing the pd dic...