auth/grpc: fix example
auth/grpc: fix example

Change-Id: I63427301914a8ae6e86a70fc0df600d02365520d
Reviewed-on: https://upspin-review.googlesource.com/5981
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin