Update install from source instructions

[hut8]

If someone wants to install from source using 1.18, they get:

$ go get -u github.com/kevinburke/go-bindata/...
go: go.mod file not found in current directory or any parent directory.
        'go get' is no longer supported outside a module.
        To build and install a command, use 'go install' with a version,
        like 'go install example.com/cmd@latest'
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.

This is what's given in the README.

A fix will be given in a PR momentarily. Thanks for maintaining this awesome tool 😄