docs: fix error handling for file-not-found

[fileformat]

I'm not a go expert, so I'm not sure exactly what the example was supposed to do before my change, but it definitely did not work.

Running on go1.17.2

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[sagikazarmark]

I believe os.ErrNotExist should only be returned when you use SetConfigFile and the file doesn't exist, so the documentation is technically correct. It might make sense to clean up the confusion with SetConfigFile.

[fileformat]

I'm a go beginner, so I may be missing something, but findConfigFile isn't behaving the way I expect and ReadInConfig is definitely returning os.ErrNotExist.

I created a gist that demonstrates what I think it should do vs what it actually does.

The output is in the gist as stderr.txt. This is with go v1.17.2.

[sagikazarmark]

There is an ongoing discussion about #1491