fix doctest syntax.

README.rst

@@ -420,7 +420,7 @@ fetching from a non existant remote::
   ...
   >>> cd(sample_buildout, 'parts', 'git-clone')
   >>> print system('echo kept > local_change')
-  >>> git remote add broken https://git.erp5.org/repos/nowhere')
+  >>> print system('git remote add broken https://git.erp5.org/repos/nowhere')
   ...
   >>> print system(buildout)
   Updating git-clone.