Fix casing of usePowerOf2Sizes

Author: epc

source/reference/command/collMod.txt

@@ -10,7 +10,7 @@ collMod
 
    :dbcommand:`collMod` makes it possible to add flags to a collection
    to modify the behavior of MongoDB. In the current release the only
-   available flag is :collflag:`usePowerof2Sizes`. The command takes the
+   available flag is :collflag:`usePowerOf2Sizes`. The command takes the
    following prototype form:
 
    .. code-block:: javascript