Update dma.h (#1678)

Fix inline doc for dma_timer_set_fraction()
diff --git a/src/rp2_common/hardware_dma/include/hardware/dma.h b/src/rp2_common/hardware_dma/include/hardware/dma.h
index 1b2d6f5..fe4b97e 100644
--- a/src/rp2_common/hardware_dma/include/hardware/dma.h
+++ b/src/rp2_common/hardware_dma/include/hardware/dma.h
@@ -863,7 +863,7 @@
  */
 bool dma_timer_is_claimed(uint timer);
 
-/*! \brief Set the divider for the given DMA timer
+/*! \brief Set the multiplier for the given DMA timer
  *  \ingroup hardware_dma
  *
  * The timer will run at the system_clock_freq * numerator / denominator, so this is the speed