drivers: spi: fix sample syntax

Upcoming versions of breathe highlight code embedded in Doxygen blocks.
In this case, the Devicetree fragment contained invalid syntax leading
to Sphinx warnings.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

include/zephyr/drivers/spi.h

@@ -163,11 +163,11 @@ struct spi_cs_control {
  * Example devicetree fragment:
  *
  * @code{.devicetree}
- *     gpio1: gpio@... { ... };
+ *     gpio1: gpio@abcd0001 { ... };
  *
- *     gpio2: gpio@... { ... };
+ *     gpio2: gpio@abcd0002 { ... };
  *
- *     spi@... {
+ *     spi@abcd0003 {
  *             compatible = "vnd,spi";
  *             cs-gpios = <&gpio1 10 GPIO_ACTIVE_LOW>,
  *                        <&gpio2 20 GPIO_ACTIVE_LOW>;