docs: README.md

Fix AT command

Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>

Author: simensrostad

README.md

@@ -78,7 +78,7 @@ west flash --erase
 1. Get the device attestation token over terminal shell:
 
    ```bash
-   at at%attesttoken?
+   at at%attesttoken
    ```
 
    *Note: Token is printed automatically on first boot of unprovisioned devices.*