Update secrets.md

Author: bebound

content/build/building/secrets.md

@@ -68,7 +68,7 @@ as a file in the build container at `/run/secrets/kube`.
 $ docker build --secret id=kube,env=KUBECONFIG .
 ```
 
-When you secrets from environment variables, you can omit the `id` parameter
+When you secrets from environment variables, you can omit the `env` parameter
 to bind the secret to a file with the same name as the variable.
 In the following example, the value of the `API_TOKEN` variable
 is mounted to `/run/secrets/API_TOKEN` in the build container.