Update README.md

we need 'go mod init first' before 'go get'

Author: trevanlye

image/multistage-builds/README.md

@@ -35,7 +35,8 @@ WORKDIR /go/src/github.com/go/helloworld/
 
 COPY app.go .
 
-RUN go get -d -v github.com/go-sql-driver/mysql \
+RUN go mod init helloworld \
+  && go get -d -v github.com/go-sql-driver/mysql \
   && CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app . \
   && cp /go/src/github.com/go/helloworld/app /root