[CSSimplify] Fix typo in CSSimplify.cpp

bellow -> below

Author: Ikko Eltociear Ashimine

lib/Sema/CSSimplify.cpp

@@ -1821,7 +1821,7 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
 
         // If this is a call to a function with a closure argument and the
         // parameter is an autoclosure, let's just increment the score here
-        // so situations like bellow are not ambiguous.
+        // so situations like below are not ambiguous.
         //    func f<T>(_: () -> T) {}
         //    func f<T>(_: @autoclosure () -> T) {}
         //