fix a bad comment in folly/Poly.h

Summary: A doc comment in folly/Poly.h makes reference to a non-existent entity "`Decay`" and is missing namespace qualifiers, making the example confusing and wrong.

Reviewed By: Alfus, yfeldblum

Differential Revision: D23038865

fbshipit-source-id: 50f515b31919552e11ae18ff645ef71812d77bfb

folly/Poly.h

@@ -219,8 +219,10 @@ struct PolyExtends : virtual I... {
  *     struct IAddable {
  *       template <class Base>
  *       struct Interface : Base {
- *         friend PolySelf<Base, Decay>
- *         operator+(PolySelf<Base> const& a, PolySelf<Base> const& b) {
+ *         friend folly::PolySelf<Base, folly::PolyDecay>
+ *         operator+(
+ *             folly::PolySelf<Base> const& a,
+ *             folly::PolySelf<Base> const& b) {
  *           return folly::poly_call<0, IAddable>(a, b);
  *         }
  *       };