fix: grammar on pitfalls page (#773)

Co-authored-by: Michael Rothgang <rothgang@math.uni-bonn.de>

Author: pevogam

templates/extras/pitfalls.md

@@ -248,7 +248,7 @@ In mathlib, `Prop` and `Bool` may be assigned different instances for typeclasse
 
 ## Not checking for distinctness
 
-Consider the following statement of the pigeonhole principle, which states that if $f : A \to B$ is a function betwen finite sets and $|A| > |B|$, then there exists two distinct elements in $A$ which $f$ map to the same element of $B$:
+Consider the following statement of the pigeonhole principle, which states that if $f : A \to B$ is a function between finite sets and $|A| > |B|$, then there exist two distinct elements in $A$ which $f$ maps to the same element of $B$:
 ```lean
 import Mathlib.Data.Fintype.Card