docs: fix manpage names being truncated

In commit d2e7377c0f, the documentation
directories were reorganized, resulting in `fname_base` (the path that
Sphinx looked for source files) to no longer expect the leading 'man/'.
This was always also stripped from the `outname`, which now strips four
characters twice over.

Fix by reducing the stripped length to only the length of the prefix in
fname_base itself.

docs/html/conf.py

@@ -277,7 +277,7 @@ man_pages = [
 # appropriate name and details
 for fname in glob.glob('man/commands/*.rst'):
     fname_base = fname[4:-4]
-    outname = 'pip-' + fname_base[13:]
+    outname = 'pip-' + fname_base[9:]
     description = u'description of {} command'.format(
         outname.replace('-', ' ')
     )