Fixed class attribute in markdown (closes #838)

plantuml/plantuml_md.py

@@ -70,7 +70,7 @@ class PlantUMLBlockProcessor(markdown.blockprocessors.BlockProcessor):
         # Generate image from PlantUML script
         imageurl = self.config['siteurl']+'/'+generate_uml_image(path, text, format)
         # Create image tag and append to the document
-        etree.SubElement(parent, "img", src=imageurl, alt=alt, classes=classes)
+        etree.SubElement(parent, "img", src=imageurl, alt=alt, attrib={'class':classes})
 
 
 # For details see https://pythonhosted.org/Markdown/extensions/api.html#extendmarkdown