fixup! Pass only Git-tracked Go files to gofumpt

Author: kyu08

CONTRIBUTING.md

@@ -109,7 +109,7 @@ by setting [`formatting.gofumpt`](https://github.com/golang/tools/blob/master/go
 To run gofumpt from your terminal go:
 
 ```
-go install mvdan.cc/gofumpt@latest && make format
+go install mvdan.cc/gofumpt@latest && git ls-files '*.go' ':!vendor' | xargs gofumpt -l -w
 ```
 
 ## Programming Font