Fix example in schema README

We should only download dependence without installing,
otherwise we'll probably get error:
go install: no install location for directory /home/qhuang/specs/schema outside GOPATH
        For more details see: go help gopath

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

schema/README.md

@@ -20,7 +20,7 @@ There is also included a simple utility for facilitating validation of a
 
 ```bash
 export GOPATH=`mktemp -d`
-go get ./...
+go get -d ./...
 go build .
 rm -rf $GOPATH
 ```