add tip about updating the c8y-tedge go-c8y-cli plugin

Signed-off-by: Reuben Miller <reuben.d.miller@gmail.com>

docs/src/extend/firmware-management/device-onboarding.md

@@ -71,6 +71,15 @@ On your machine (not the target device that will be on-boarded) you will need on
  c8y extension install thin-edge/c8y-tedge
  ```
 
+ :::tip
+
+ If you have already installed the `c8y-tedge` go-c8y-cli extension, then make sure it is up-to-date using:
+
+ ```sh
+ c8y extension update tedge
+ ```
+ :::
+
 3. Create a Cumulocity session file (if one does not already for the Cumulocity instance you wish to connect to)
 
  ```sh