fix: teeny typo (#130)

Author: devneill

exercises/03.compound-components/02.solution.validation/README.mdx

@@ -2,5 +2,5 @@
 
 👨‍💼 Runtime validation isn't the best (it would be better if we could enforce
 this statically via TypeScript), but unfortunately it's the best we can do with
-the composition model offered by React. That said, we it's unlikely people will
+the composition model offered by React. That said, it's unlikely people will
 mess this up now that we have this runtime validation in place.