exp/cmd/upbox: fix typo in doc comment

Change-Id: Ia5412b847afc3f00415b1bafcee82620101b3fee
Reviewed-on: https://upspin-review.googlesource.com/10060
Reviewed-by: David Presotto <presotto@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/cmd/upbox/main.go b/cmd/upbox/main.go
index fbbda7a..ad510d9 100644
--- a/cmd/upbox/main.go
+++ b/cmd/upbox/main.go
@@ -24,7 +24,7 @@
 	  flags:
 	    debug: cockroach
 	keyserver: key.uspin.io
-	domain: exmaple.com
+	domain: example.com
 
 
 The Users and Servers lists specify the users and servers to create within this