Clarify LIFO vs FIFO

The last commit used the wrong verbiage in a comment. This fixes it and
clarifies that MutableMapping uses FIFO.

Author: Andrew5057

Lib/collections/__init__.py

@@ -1255,7 +1255,7 @@ def copy(self):
 
 
     # This method has a default implementation in MutableMapping, but dict's
-    # equivalent is first-in, first-out.
+    # equivalent is last-in, first-out instead of first-in, first-out.
     def popitem(self):
         return self.data.popitem()