docs: use go install to install build-lambda-zip. (#464)

go 1.18 removed the ability to use `go get` to install binaries.
we should use `go install` instead.
fixes #463

README.md

@@ -58,8 +58,7 @@ Windows developers may have trouble producing a zip file that marks the binary a
 
 Get the tool
 ``` shell
-set GO111MODULE=on
-go.exe get -u github.com/aws/aws-lambda-go/cmd/build-lambda-zip
+go.exe install github.com/aws/aws-lambda-go/cmd/build-lambda-zip@latest
 ```
 
 Use the tool from your `GOPATH`. If you have a default installation of Go, the tool will be in `%USERPROFILE%\Go\bin`.