Update cookbook.md

Renamed frame to pc_frame (review feedback)

Author: martindevans

wrappers/csharp/Documentation/cookbook.md

@@ -281,8 +281,8 @@ pipe.Start();
 
 using (var frames = pipe.WaitForFrames())
 using (var depth = frames.DepthFrame)
-using (var frame = pc.Process(depth))
-using (var points = frame.As<Points>())
+using (var pc_frame = pc.Process(depth))
+using (var points = pc_frame.As<Points>())
 {
     // CopyVertices is extensible, any of these will do:
     var vertices = new float[points.Count * 3];
@@ -424,4 +424,4 @@ using (var sensor = playback.Sensors[0])
     SpinWait.SpinUntil(() => playback.Status == PlaybackStatus.Stopped);
     Print();
 }
-```
+```