dt-bindings: clk: fix example for single-output provider

As described above single-output clock provider should have
0 cells number, so let's fix it by using 0 as cells number.
Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/clock/clock-bindings.txt

@@ -94,7 +94,7 @@ clock is connected to output 0 of the &ref.
     /* external oscillator */
     osc: oscillator {
         compatible = "fixed-clock";
-        #clock-cells = <1>;
+        #clock-cells = <0>;
         clock-frequency  = <32678>;
         clock-output-names = "osc";
     };