Fix build instructions

[Weltraumschaf]

The documented way with go get gives an error message:

go get github.com/zmap/zgrab2 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'.

I'm not very experienced with go. Worked works for me: Clone repo and run make.

[Weltraumschaf]

Hi, what's up?

[developStorm]

Sorry for the delay. Thanks for helping us with the documentation!