doc: fix wrong header guard code comment

Copy/Paste leftover from openthread POSIX platform.

Signed-off-by: Marcel Graber <[email protected]>

Author: marcel-cd

modules/openthread/platform/platform-zephyr.h

@@ -117,4 +117,4 @@ int notify_new_rx_frame(struct net_pkt *pkt);
  */
 int notify_new_tx_frame(struct net_pkt *pkt);
 
-#endif  /* PLATFORM_POSIX_H_ */
+#endif /* PLATFORM_ZEPHYR_H_ */