docs: Use valid docker bind propagation field

Co-Authored-By: notnoop <[email protected]>

website/source/docs/drivers/docker.html.md

@@ -324,7 +324,7 @@ The `docker` driver supports the following configuration in the job spec.  Only
           source = "/path/in/host"
           readonly = false
           bind_options {
-            propagation = "xxxx"
+            propagation = "rshared"
           }
         },
         # sample tmpfs mount