Fix typo

Author: d10n

source/reference/method/db.collection.totalSize.txt

@@ -13,4 +13,4 @@ db.collection.totalSize()
 .. method:: db.collection.totalSize()
 
    :returns: The total size in bytes of the data in the collection plus
-             the size of every indexes on the collection.
+             the size of every index on the collection.