contrib: labgrid-webapp: Fix minor docs issues

Add a link to graphviz and drop the unwanted --help from the invocation
line.

Signed-off-by: Simon Glass <sjg@chromium.org>

Author: sjg20

contrib/README.rst

@@ -24,7 +24,7 @@ Quick Start
      --port PORT           Port to serve on
      --proxy PROXY, -P PROXY
 
-   venv $ ./contrib/labgrid-webapp --help
+   venv $ ./contrib/labgrid-webapp
    INFO:     Available routes:
    INFO:       - /labgrid/graph
    INFO:     Started server process [2378028]
@@ -34,7 +34,7 @@ Quick Start
    ...
 
 Please note that the graph feature relies on a valid `graphviz` system
-installation.
+installation. See http://graphviz.org/download/
 
 By default the application will start on port 8800.