Fix doc error on displayable surface types

VdpVideoSurface is not directly displayable in the current
implementation of VDPAU. VdpOutputSurface is. Make the documentation
consistent with reality.
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>

include/vdpau/vdpau.h

@@ -1704,7 +1704,7 @@ typedef VdpStatus VdpVideoSurfacePutBitsYCbCr(
  * - The Hardware that implements \ref VdpPresentationQueue
  *   "VdpPresentationQueue" functionality,
  *
- * VdpVideoSurfaces are directly displayable using a \ref
+ * VdpOutputSurfaces are directly displayable using a \ref
  * VdpPresentationQueue "VdpPresentationQueue" object.
  *
  * @{