Dream.run docs: remove mention of ~stop_on_input

It was removed in #46, but a dangling reference to it still remained.
Caught by Yawar Amin in

  #46 (comment)

src/dream.mli

@@ -2122,7 +2122,7 @@ val run :
     - [~stop] is a promise that causes the server to stop accepting new
       requests, and {!Dream.run} to return. Requests that have already entered
       the Web application continue to be processed. The default value is a
-      promise that never resolves. However, see also [~stop_on_input].
+      promise that never resolves.
     - [~error_handler] handles all errors, both from the application, and
       low-level errors. See {!section-errors} and example
       {{:https://github.com/aantron/dream/tree/master/example/9-error#files}