Change apostrophe placement in advanced.rst

See Rule2a on http://www.grammarbook.com/punctuation/apostro.asp

docs/user/advanced.rst

@@ -227,7 +227,7 @@ By default, ``verify`` is set to True. Option ``verify`` only applies to host ce
 
 You can also specify a local cert to use as client side certificate, as a single
 file (containing the private key and the certificate) or as a tuple of both
-file's path::
+files' paths::
 
     >>> requests.get('https://kennethreitz.org', cert=('/path/client.cert', '/path/client.key'))
     <Response [200]>