go vet issues in template_test.go

[shawnps]

I'm not sure what the string should be otherwise I would have made the PR myself, but there are multiple go vet issues in protoc-gen-swagger/genswagger/template_test.go:

https://goreportcard.com/report/github.com/gengo/grpc-gateway#go_vet

[shawnps]

(Maybe just remove the file argument in the Errorf calls)

[achew22]

I'm the author on those. I absently mindedly wrote "file" in there when I wanted to write %s. Thanks for doing all this cleanup!

[shawnps]

@achew22 np. If you like what I found, please consider adding a Go Report Card badge to the README 👍

[![Go Report Card](https://goreportcard.com/badge/github.com/gengo/grpc-gateway)](https://goreportcard.com/report/github.com/gengo/grpc-gateway)