key_file should be

According to https://azure.microsoft.com/en-us/documentation/articles/cloud-services-python-how-to-use-service-management/ key_file should be '/path/to/azure_cert.pem'
azure_cert.cer raises ssl.SSLError: [SSL] PEM lib (_ssl.c:2580) error.

Closes #611

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
diff --git a/docs/examples/compute/azure/instantiate.py b/docs/examples/compute/azure/instantiate.py
index 3ccf548..ce97a9e 100644
--- a/docs/examples/compute/azure/instantiate.py
+++ b/docs/examples/compute/azure/instantiate.py
@@ -3,4 +3,4 @@
 
 cls = get_driver(Provider.AZURE)
 driver = cls(subscription_id='subscription-id',
-             key_file='/path/to/azure_cert.cer')
+             key_file='/path/to/azure_cert.pem')