Fix build error on Python 2.6

This was seen on winbot[*]:

  ValueError: path 'docs/_build/' cannot end with '/'

with a traceback ending in "c:\Python26_32\lib\distutils\util.py", line
201, in convert_path.

[*] http://winbot.zope.org/builders/transaction_py_265_32/builds/845/steps/buildout/logs/stdio

I cannot reproduce the error locally, using Python 2.6.9.