Merge pull request #27978 from MartinKanters/fix-variable-substitution-typo

Fixed variable substitution typo

Author: k8s-ci-robot

content/en/docs/contribute/generate-ref-docs/kubernetes-api.md

@@ -90,8 +90,8 @@ This section shows how to generate the
 For example:
 
 ```shell
-export K8S_WEBROOT=$(GOPATH)/src/github.com/<your-username>/website
-export K8S_ROOT=$(GOPATH)/src/k8s.io/kubernetes
+export K8S_WEBROOT=${GOPATH}/src/github.com/<your-username>/website
+export K8S_ROOT=${GOPATH}/src/k8s.io/kubernetes
 export K8S_RELEASE=1.17.0
 ```