Fix typos in code comments (#1874)

core/src/typecheck/mod.rs

@@ -2147,8 +2147,8 @@ fn check<V: TypecheckVisitor>(
  // case, once we have inferred an actual type for this field, we need to unify
  // what's inside the environment with the actual type to ensure that they agree.
  //
- // `need_unif_step` store the list of fields corresponding to the case 2, which
- // requires this additional unification step. Note that performing the additional
+ // `need_unif_step` stores the list of fields corresponding to the case 2, which
+ // require this additional unification step. Note that performing the additional
  // unification in case 1. should be harmless, but it's wasteful, and is also not
  // entirely trivial because of polymorphism (we need to make sure to instantiate
  // polymorphic type annotations). So it's simpler to just skip it in this case.