Fix type checking brief explanation

Author: spastorino

src/overview.md

@@ -73,7 +73,9 @@ we'll talk about that later.
   of pairing up an impl with each reference to a trait), and [type
   checking] (the process of converting the types found in the HIR
   (`hir::Ty`), which represent the syntactic things that the user wrote,
-  into the internal representation used by the compiler (`Ty<'tcx>`)).
+  into the internal representation used by the compiler (`Ty<'tcx>`),
+  and use that information to verify the type safety, correctness and
+  coherence of the types used in the program).
 - The HIR is then [lowered to Mid-Level Intermediate Representation (MIR)][mir].
   - Along the way, we construct the THIR, which is an even more desugared HIR.
     THIR is used for pattern and exhaustiveness checking. It is also more