[docs] Added missing reference to 'diff' in 'From numpy to xtensor'
[docs] Added missing reference to 'diff' in 'From numpy to xtensor'

JohanMabille/xtensor,xtensor-stack/xtensor,xtensor-stack/xtensor,QuantStack/xtensor,QuantStack/xtensor,JohanMabille/xtensor