Minor README Fix for New Versions of Go

[aolszowka]

Attempting to follow the README as written results in:

>go get github.com/bemasher/rtlamr-collect
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'.

Make the minor README change to reflect the modification to go install

[taciturnaxolotl]

Just saw your branch after I made the same fix. I'll delete my changes. I also opened an issue for the request so I'll link it to be closed.
Fixes #43