clean up kube-root references in docs

Author: BenTheElder

site/content/docs/user/known-issues.md

@@ -127,7 +127,6 @@ Flags:
       --base-image string   name:tag of the base image to use for the build (default "kindest/base:v20181203-d055041")
   -h, --help                help for node-image
       --image string        name:tag of the resulting image to be built (default "kindest/node:latest")
-      --kube-root string    Path to the Kubernetes source directory (if empty, the path is autodetected)
       --type string         build type, default is docker (default "docker")
 
 Global Flags:

site/content/docs/user/working-offline.md

@@ -109,7 +109,7 @@ You can clone Kubernetes source code.
 ### Building image
 
 ```sh
-➜  ~ kind build node-image --image kindest/node:main --kube-root $GOPATH/src/k8s.io/kubernetes 
+➜  ~ kind build node-image --image kindest/node:main $GOPATH/src/k8s.io/kubernetes 
 Starting to build Kubernetes
 ...
 Image build completed.