Missed instance of "white-listed"

[klonos]

We missed this in #4441: https://github.com/backdrop/backdrop/blob/1.x/core/includes/common.inc#L2806

 * ...
 * attribute names, it is strongly recommended to allow only white-listed names,
 * ...

[klonos]

PR here: https://github.com/backdrop/backdrop/pull/4703 ...nothing to test, since this changes only the wording within a docblock - just needs review of the rewording.

[stpaultim]

I think I can do code review on this one. ;-)

[quicksketch]

Merged into 1.x and 1.27.x. Thanks @klonos and @stpaultim!