Consistency between `historyRedo` and `historyUndo`?

[aphillips]

6.1.2 Attributes
https://w3c.github.io/input-events/#interface-InputEvent-Attributes

to redo the last undone editing action

historyRedo says "to redo" but historyUndo just says "undo". Edit to make consistent?

[This issue was found in the I18N horizontal review, but is not an internationalization issue, hence the tracker label.]