Fixed a typo

--HG--
branch : trunk
diff --git a/jinja2/environment.py b/jinja2/environment.py
index 4a9c9d1..9d43339 100644
--- a/jinja2/environment.py
+++ b/jinja2/environment.py
@@ -53,7 +53,7 @@
 def copy_cache(cache):
     """Create an empty copy of the given cache."""
     if cache is None:
-        return Noe
+        return None
     elif type(cache) is dict:
         return {}
     return LRUCache(cache.capacity)