README build instructions update

[johngillott]

Hello 👋

I attempted to install misspell following instructions provided in the README and encountered the following error:

go get -u github.com/client9/misspell/cmd/misspell
go get: installing executables with 'go get' in module mode is deprecated.
        Use 'go install pkg@version' instead.
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.

It may be worth updating the README install section to follow the recommended workflow for installing Go libraries which changed in 1.16. The install instruction would now be go install github.com/client9/misspell/cmd/misspell@latest