Fix reference to bootstrap.py

There is no bootstrap/ subdirectory in git.

Then again there's no bootstrap/ subdirectory nor bootstrap.py nor, in fact, buildout.cfg in https://pypi.python.org/packages/source/Z/Zope2/Zope2-2.13.22.zip, so I don't know what this document is talking about.  Broken MANIFEST.in?

doc/INSTALL.rst

@@ -65,7 +65,7 @@ On Linux, this can be done as follows::
   $ wget http://pypi.python.org/packages/source/Z/Zope2/Zope2-<Zope version>.tar.gz
   $ tar xfvz Zope2-<Zope version>.tar.gz
   $ cd Zope2-<Zope version>
-  $ /path/to/your/python bootstrap/bootstrap.py
+  $ /path/to/your/python bootstrap.py
   $ bin/buildout