Fix file mode in image.save examples

Signed-off-by: Joffrey F <joffrey@docker.com>

docker/api/image.py

@@ -32,7 +32,7 @@ class ImageApiMixin(object):
         Example:
 
             >>> image = cli.get_image("busybox:latest")
-            >>> f = open('/tmp/busybox-latest.tar', 'w')
+            >>> f = open('/tmp/busybox-latest.tar', 'wb')
             >>> for chunk in image:
             >>>   f.write(chunk)
             >>> f.close()

docker/models/images.py

@@ -84,7 +84,7 @@ class Image(Model):
         Example:
 
             >>> image = cli.get_image("busybox:latest")
-            >>> f = open('/tmp/busybox-latest.tar', 'w')
+            >>> f = open('/tmp/busybox-latest.tar', 'wb')
             >>> for chunk in image:
             >>>   f.write(chunk)
             >>> f.close()