Update "document_root" to match new layout/naming.

examples/websocket/websocket.c

@@ -32,7 +32,7 @@ int main(void)
     struct mg_callbacks callbacks;
     const char *options[] = {
         "listening_ports", "8080",
-        "document_root", "websocket_html_root",
+        "document_root", "docroot",
         NULL
     };