Ignore E902 (directory doesn't exist) error when linting docs/examples/
directory.

This directory is not included in the distribution tarball so we don't want lint
to fail there.
diff --git a/tox.ini b/tox.ini
index 428b2ab..d3d9f23 100644
--- a/tox.ini
+++ b/tox.ini
@@ -50,4 +50,4 @@
 commands = flake8 --exclude="test/" libcloud/
            flake8 --max-line-length=160 libcloud/test/
            flake8 demos/
-           flake8 docs/examples/
+           flake8 --ignore=E902 docs/examples/