Fixes a fatal edoc error. Changes edoc spec to init_routes/1

The edoc spec for <code>init_routes/1</code> is incorrect, causing a fatal error.

<pre>
computer:webmachine me$ make edoc
./src/webmachine_router.erl, function init_routes/1: at line 110: @spec arity does not match.
edoc: skipping source file './src/webmachine_router.erl': {'EXIT',error}.
edoc: error in doclet 'edoc_doclet': {'EXIT',error}.
edoc: edoc terminated abnormally: error.
make: *** [edoc] Error 1
</pre>

This corrects the issue.