refactor: go-vet alert fix.

[Alt-NoRock]

In go-vet, Fprintln function is not allowed below context:

• possible formatting directive %f
• redundant newline
  Thus, use Fprintf instead of Fprintln.

[Alt-NoRock]

Short Description

I want to add new defined format.
In fmts/README.md, we need to go test ./... before PR. but this cmd failed.

My Environment

VSCodeSpace used

$ uname -a
Linux codespaces-0ec3fa 5.15.0-1042-azure #49-Ubuntu SMP Tue Jul 11 17:28:46 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
$ go version
go version go1.20.5 linux/amd64
$ git branch
master

Error Log

add new errorformat test failed by go vet

$ go test ./...
ok      github.com/reviewdog/errorformat        0.008s
# github.com/reviewdog/errorformat/cmd/errorformat
cmd/errorformat/main.go:70:2: fmt.Fprintln call has possible formatting directive %f
cmd/errorformat/main.go:70:2: fmt.Fprintln arg list ends with redundant newline
FAIL    github.com/reviewdog/errorformat/cmd/errorformat [build failed]
ok      github.com/reviewdog/errorformat/fmts   0.120s
ok      github.com/reviewdog/errorformat/writer 0.004s
FAIL

if without go vet, test success.

$ go test -vet=off ./...
ok      github.com/reviewdog/errorformat        0.010s
ok      github.com/reviewdog/errorformat/cmd/errorformat        0.003s
ok      github.com/reviewdog/errorformat/fmts   0.122s
ok      github.com/reviewdog/errorformat/writer 0.004s

Reason of bug and countermeasure.

In usage function, use Fprintf instead of Fprintln.
Because usage context includes formatting directive %f:

Example:
	$ echo '/path/to/file:14:28: error message\nfile2:3:4: msg' | errorformat "%f:%l:%c: %m"

[shogo82148]

LGTM

[review-dog]

Hi, @Alt-NoRock! We merged your PR to reviewdog! 🐶
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