PreferencesDialog: make whitespace strings consistent

[davidmhewitt]

We use "whitespace" here, so this should maybe be the same? Or maybe that one should be changed? Or maybe they should both have a hyphen? 😆

But I think they should at least be consistent.

code/src/Dialogs/PreferencesDialog.vala

Line 25 in a03ec88

general_box.add (new SettingSwitch (_("Strip trailing whitespace on save"), "strip-trailing-on-save"));

[danirabbit]

Looked around to try to find some kind of more definitive answer or styleguide here but the best I could find is that generally "whitespace" (as a single word) can refer to any character that is not visible (like tab or space) whereas "white space" refers more generally to the concept of leaving a blank space. So single word is probably appropriate here since we're talking making whitespace characters visible and stripping trailing whitespace characters