dm: Fix documentation entry as there is no UCLASS_CLOCK uclass

There is no UCLASS_CLOCK uclass defined. Instead we do use the UCLASS_CLK.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
diff --git a/include/clk.h b/include/clk.h
index d24e997..a909b71 100644
--- a/include/clk.h
+++ b/include/clk.h
@@ -20,7 +20,7 @@
  * clock provider. This API provides a standard means for drivers to enable and
  * disable clocks, and to set the rate at which they oscillate.
  *
- * A driver that implements UCLASS_CLOCK is a clock provider. A provider will
+ * A driver that implements UCLASS_CLK is a clock provider. A provider will
  * often implement multiple separate clocks, since the hardware it manages
  * often has this capability. clk-uclass.h describes the interface which
  * clock providers must implement.