Update adapter discovery docs (zigbee2mqtt#639)

Author: Koenkk

README.md

@@ -49,7 +49,7 @@
 1. Wait till Zigbee2MQTT starts and press **OPEN WEB UI** to verify Zigbee2MQTT started correctly.
    - If it shows `502: Bad Gateway` wait a bit more and refresh the page.
    - If this takes too long (e.g. 2 minutes +) check the **Log** tab to see what went wrong.
-   - In case the add-on fails to start with the following error: `Error: No path provided and failed to auto detect path`, we need to fill in the `serial` section (which we skipped in step 5). Format can be found [here](https://www.zigbee2mqtt.io/guide/configuration/adapter-settings.html#adapter-settings), but skip the initial `serial:` indent. e.g.: <br>
+   - In case the add-on fails to start with the following error: `USB adapter discovery error (No valid USB adapter found). Specify valid 'adapter' and 'port' in your configuration.`, we need to fill in the `serial` section (which we skipped in step 5). Format can be found [here](https://www.zigbee2mqtt.io/guide/configuration/adapter-settings.html#adapter-settings), but skip the initial `serial:` indent. e.g.: <br>
      ```yaml
      port: /dev/serial/by-id/usb-Texas_Instruments_TI_CC2531_USB_CDC___0X00124B0018ED3DDF-if00
      adapter: zstack