fix(pitfalls.md): remove repeated word

[AxelBoldt]

In pitfalls.md / Distance in Fin n → ℝ, in the sentence

To instead use the standard Euclidean metric (also called the $L^2$ metric), you must use instead use EuclideanSpace ℝ (Fin n)

I removed the double "use" and the double "instead".

[grunweg]

Thanks!