dt-bindings: h8300 clocksource: correct spelling of pulse

Correct what appears to be a typo in the spelling of pulse.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/timer/renesas,tpu.txt

-* Renesas H8/300 Timer Pluse Unit
+* Renesas H8/300 Timer Pulse Unit
 
 The TPU is a 16bit timer/counter with configurable clock inputs and
 programmable compare match.