Use os.UserHomeDir() in document

[micnncim]

Related to #853, replaced mitchellh/go-homedir with os.UserHomeDir() in README.md.

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[umarcor]

Hi @micnncim! I am not sure about this PR making sense on its own.

In #853, this change is applied along with making the codebase compatible for go 1.10, 1.11 and 1.12.

In #876, this change is applied along with deprecating go 1.10 and 1.11, thus making cobra >=1.12 only.

Since this PR replaces mitchellh/go-homedir with os.UserHomeDir without further modifications, I assume that you agree with #876. If so, I'd suggest to close this issue and review #876 instead.

[micnncim]

Thank you @umarcor! I don't notice #876 so will close this PR.