Update README.md

Co-authored-by: Pauline Ribeyre <4224001+paulineribeyre@users.noreply.github.com>

README.md

@@ -524,7 +524,7 @@ fence-create client-modify --client CLIENT_NAME --urls http://localhost/api/v0/o
 That command should output any modifications to the client. Similarly, multiple URLs are
 allowed here too.
 
-Add `--append` argument to add new callback urls or allowed scopes to existing client using `--append --urls` or `--append --allowed-scopes`
+Add `--append` argument to add new callback urls or allowed scopes to existing client (instead of replacing them) using `--append --urls` or `--append --allowed-scopes`
 ```bash
 fence-create client-modify --client CLIENT_NAME --urls http://localhost/api/v0/new/oauth2/authorize --append
 ```