Docs: Fix typo in Preference android:order attribute.

        <!-- The summary for the Preference in a PreferenceActivity screen. -->

        <attr name="summary" />

        <!-- The order for the Preference (lower values are to be ordered first). If this is not

             specified, the default orderin will be alphabetic. -->

             specified, the default ordering will be alphabetic. -->

        <attr name="order" format="integer" />

        <!-- When used inside of a modern PreferenceActivity, this declares

             a new PreferenceFragment to be shown when the user selects this item. -->