Fix example of using setuptools find_links
Fix example of using setuptools find_links

setuptools.package_index.local_open is used for file: URLs, and only
handles directories if the URL ends with a slash.  Add the trailing
slash to pip's documentation to reduce confusion.

fiber-space/pip,RonnyPfannschmidt/pip,RonnyPfannschmidt/pip,ncoghlan/pip,xavfernandez/pip,rouge8/pip,ncoghlan/pip,natefoo/pip,alex/pip,pradyunsg/pip,zvezdan/pip,willingc/pip,esc/pip,dstufft/pip,willingc/pip,sbidoul/pip,mindw/pip,rbtcollins/pip,techtonik/pip,alex/pip,pfmoore/pip,pypa/pip,techtonik/pip,willingc/pip,esc/pip,sigmavirus24/pip,esc/pip,sigmavirus24/pip,pypa/pip,RonnyPfannschmidt/pip,rouge8/pip,sbidoul/pip,benesch/pip,pradyunsg/pip,techtonik/pip,fiber-space/pip,rbtcollins/pip,benesch/pip,zvezdan/pip,natefoo/pip,fiber-space/pip,alex/pip,nthall/pip,nthall/pip,sigmavirus24/pip,rbtcollins/pip,xavfernandez/pip,pfmoore/pip,dstufft/pip,benesch/pip,jythontools/pip,atdaemon/pip,atdaemon/pip,nthall/pip,dstufft/pip,jythontools/pip,zvezdan/pip,mindw/pip,natefoo/pip,ncoghlan/pip,rouge8/pip,jythontools/pip,atdaemon/pip,mindw/pip,xavfernandez/pip