Fix DiverseListImpl comment grammar

Author: danra

include/swift/Basic/DiverseList.h

@@ -127,7 +127,7 @@ class DiverseListBase {
 
 /// An "abstract" base class for DiverseList<T> which does not
 /// explicitly the preferred inline capacity.  Most of the
-/// implementation is on this class.
+/// implementation is in this class.
 template <class T> class DiverseListImpl : private DiverseListBase {
   DiverseListImpl(const DiverseListImpl<T> &other) = delete;
   DiverseListImpl(DiverseListImpl<T> &&other) = delete;