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.

docs/reference/pip_install.rst

@@ -513,7 +513,7 @@ To have the dependency located from a local directory and not crawl PyPI, add th
 
   [easy_install]
   allow_hosts = ''
-  find_links = file:///path/to/local/archives
+  find_links = file:///path/to/local/archives/
 
 
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