X.H.DynamicLog: Fix xmobarProp example
X.H.DynamicLog: Fix xmobarProp example

In [1] we changed the return type to not be an IO action and hence this
example can't work, even with an otherwise correct configuration.

[1]: 168cb6a6c3a774bc05032536e37b982cd6b4d7cc (Removed unnecessary IO)

xmonad/xmonad-contrib,xmonad/xmonad-contrib