dt-bindings: arm-boards: Update pointer to ARM CPU bindings

The ARM CPU DT bindings were converted from plain text to YAML, but not
all referrers were updated.

Fixes: 672951cb ("dt-bindings: arm: Convert cpu binding to json-schema")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/arm-boards

@@ -199,7 +199,7 @@ The description for the board must include:
      A detailed description of the bindings used for "psci" nodes is present
      in the psci.yaml file.
    - a "cpus" node describing the available cores and their associated
-     "enable-method"s. For more details see cpus.txt file.
+     "enable-method"s. For more details see cpus.yaml file.
 
 Example: