exp/cmd/demoserver: remove mention of 'dingus' in error  message

During development this server was named 'dingus',
but was later renamed. I missed this error while renaming.

Change-Id: I0fed59a62b377a6753e8c0c7cd5be157a544a3f6
Reviewed-on: https://upspin-review.googlesource.com/9460
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/cmd/demoserver/main.go b/cmd/demoserver/main.go
index 4e9565b..786a6fc 100644
--- a/cmd/demoserver/main.go
+++ b/cmd/demoserver/main.go
@@ -348,7 +348,7 @@
 
 // The DirServer and StoreServer methods below are not implemented.
 
-var errNotImplemented = errors.E(errors.Permission, errors.Str("method not implemented: dingus is read-only"))
+var errNotImplemented = errors.E(errors.Permission, errors.Str("method not implemented: demoserver is read-only"))
 
 func (*dirServer) Put(entry *upspin.DirEntry) (*upspin.DirEntry, error) {
 	return nil, errNotImplemented