fix(pitfalls.md): remove repeated word (#760)

In [pitfalls.md / Distance in Fin n →
ℝ](https://leanprover-community.github.io/extras/pitfalls.html#distance-in-fin-n--%E2%84%9D),
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".

Author: AxelBoldt

templates/extras/pitfalls.md

@@ -453,7 +453,7 @@ example : dist ![(1 : ℝ),0] ![0,1] = 1 := by
     fin_cases b <;> simp
 ```
 
-To instead use the standard Euclidean metric (also called the $L^2$ metric), you must use instead use `EuclideanSpace ℝ (Fin n)`, which is an abbreviation for `WithLp 2 (Fin n → ℝ)`. Vectors of this type can be written using the `!₂[x,y,z,...]` notation.
+To instead use the standard Euclidean metric (also called the $L^2$ metric), you must use `EuclideanSpace ℝ (Fin n)`, which is an abbreviation for `WithLp 2 (Fin n → ℝ)`. Vectors of this type can be written using the `!₂[x,y,z,...]` notation.
 ```lean
 example : dist !₂[(1 : ℝ),0] !₂[0,1] = √2 := by
   norm_num [EuclideanSpace.dist_eq]