Fix typos and one extra import.

--HG--
branch : trunk
diff --git a/jinja2/environment.py b/jinja2/environment.py
index 75f58d6..a08aad2 100644
--- a/jinja2/environment.py
+++ b/jinja2/environment.py
@@ -288,8 +288,8 @@
                 loader=missing, cache_size=missing, auto_reload=missing,
                 bytecode_cache=missing):
         """Create a new overlay environment that shares all the data with the
-        current environment except of cache and the overriden attributes.
-        Extensions cannot be removed for a overlayed environment.  A overlayed
+        current environment except of cache and the overridden attributes.
+        Extensions cannot be removed for an overlayed environment.  An overlayed
         environment automatically gets all the extensions of the environment it
         is linked to plus optional extra extensions.