vdpau.h: Clarify video mixer field amount recommendation

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>

include/vdpau/vdpau.h

@@ -474,7 +474,7 @@
  * determining the minimum required number of surfaces for optimal operation,
  * and the maximum number of useful surfaces, beyond which surfaces are not
  * used. It is recommended that in all cases other than plain bob/weave, at
- * least 2 past and 1 future frame be provided.
+ * least 2 past and 1 future field be provided.
  *
  * Note that it is entirely possible, in general, for any of the
  * \ref VdpVideoMixer "VdpVideoMixer" post-processing steps other than