doc: remove extraneous "on"

Fixes: #2000
PR-URL: #2010
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jamie Davis <davisjam@vt.edu>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

docs/src/design.rst

@@ -126,7 +126,7 @@ so the current approach is to run blocking file I/O operations in a thread pool.
 For a thorough explanation of the cross-platform file I/O landscape, checkout
 `this post <http://blog.libtorrent.org/2012/10/asynchronous-disk-io/>`_.
 
-libuv currently uses a global thread pool on which all loops can queue work on. 3 types of
+libuv currently uses a global thread pool on which all loops can queue work. 3 types of
 operations are currently run on this pool:
 
     * File system operations