Fix simple typo: properpty -> property (#717)

Closes #716

Author: timgates42

pipeline/forms.py

@@ -136,7 +136,7 @@ def __new__(cls, name, bases, attrs):
         # If we define any packages, we'll need to use our special
         # PipelineFormMediaProperty class. We use this instead of intercepting
         # in __getattribute__ because Django does not access them through
-        # normal properpty access. Instead, grabs the Media class's __dict__
+        # normal property access. Instead, grabs the Media class's __dict__
         # and accesses them from there. By using these special properties, we
         # can handle direct access (Media.css) and dictionary-based access
         # (Media.__dict__['css']).