cmd/stringer: fixed usage line typo
cmd/stringer: fixed usage line typo

Change-Id: I5852d03c0326d82c13d9e1078f9648ecc695fbbf
Reviewed-on: https://go-review.googlesource.com/22150
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,golang/tools