docs: change to working tox command

"tox run -e exec" didn't work for me. It complained about "run" being an invalid subcommand. "tox -e exec" did work though.

README.rst

@@ -49,7 +49,7 @@ To set up a configuration for development, start by copying the example configur
 
 To build and run the server (for development only)::
 
-    tox run -e exec
+    tox -e exec
 
 In your browser, open http://localhost:8080/ to reach the server. Log in as any user in the ``conf/directory.yaml`` directory file. In the ``conf/imsd.conf`` sample configuration file, the users ``Hardware`` and ``Loosy`` are administrators, and in the sample directory, all users have passwords that match their handles. You'll want to log in as one of those to set up an Event.