The typo is a grammatical error I found in the Help Overlay description under "Information Layers Button" while playing the 100 Years War campaign: In the first sentence of the text, right after "Selecting one..", its followed by "one those buttons toggles information..." [see example below]

A slight correction of the grammatical structure in the sentence could be done in the format of adding the word "of" in between "one" and "those"; or, the sentence could be reworded as follows: "The buttons seen below are the Information Layer Buttons, and can be used to toggle specific information shown on the world map. Multiple layers can be active at the same time."

Hope this helps!