display-night-light: Change ‘top bar’ to ‘header bar’ for clarity

‘Top bar’ is used elsewhere in the documentation to refer to the bar provided by gnome-shell, whereas here it’s used to mean the header bar of the Displays settings panel.

Avoid the confusion by using ‘header bar’ instead, as used elsewhere in the documentation in similar situations (for example, in the Wacom tablet settings panel).

Signed-off-by: Philip Withnall pwithnall@endlessos.org