docs($q): correct count from three to two differences in the 'Difference...

[robinboehm]

At the $q doc is a listing with the differences between Q and $q

"There are three main differences"
But only a list with two items.

Change wording from "three" -> "two"

[mary-poppins]

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[robinboehm]

I've already signed the CLA

fyi

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