spi: dt-bindings: spi-controller: correct example indentation

Example DTS mixed two with four-space indentation.  Preferred is four
spaces, for readability.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220722190910.76865-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-controller.yaml

@@ -139,9 +139,9 @@ examples:
         };
 
         flash@2 {
-          compatible = "jedec,spi-nor";
-          spi-max-frequency = <50000000>;
-          reg = <2>, <3>;
-          stacked-memories = /bits/ 64 <0x10000000 0x10000000>;
+            compatible = "jedec,spi-nor";
+            spi-max-frequency = <50000000>;
+            reg = <2>, <3>;
+            stacked-memories = /bits/ 64 <0x10000000 0x10000000>;
         };
     };