Update DMAChannel.h

I'm planning to merge the teensyduino 1.20 branch of DmaSpi to master tomorrow and then you can remove that hint, or update it please.

teensy3/DMAChannel.h

 // https://github.com/pedvide/ADC
 // https://github.com/duff2013/SerialEvent
 // https://github.com/pixelmatix/SmartMatrix
-// https://github.com/crteensy/DmaSpi
+// https://github.com/crteensy/DmaSpi <-- DmaSpi has adopted this scheme
 
 #ifdef __cplusplus