menu.xml: replace odd <execute> tag with <command>

[johanmalm]

Increase consistency by using tags throughout menu.xml file.

The openbox.org wiki says that the tag is depreciated.

http://openbox.org/wiki/Help:Actions#Action_syntax

Same as c217aa3c on helium branch

[johanmalm]

Looks like we missed this one.

[johnraff]

Thanks - good catch!