docs: Fixed action bar example. Bug: 13806963

<resources>

    <!-- the theme applied to the application or activity -->

    <style name="CustomActionBarTheme"

           parent="@style/Theme.Holo.Light.DarkActionBar">

           parent="@android:style/Theme.Holo.Light.DarkActionBar">

        <item name="android:actionBarStyle">@style/MyActionBar</item>

    </style>

    <!-- ActionBar styles -->

    <style name="MyActionBar"

           parent="@style/Widget.Holo.Light.ActionBar.Solid.Inverse">

           parent="@android:style/Widget.Holo.Light.ActionBar.Solid.Inverse">

        <item name="android:background">@drawable/actionbar_background</item>

    </style>

</resources>