Update stringtable.xml

Added some terms in spanish.
This is supposed to be the description of the Kestrel item. Should be something like "Anemómetro balístico" o "Anemómetro digital" (http://www.weathercontrols.com/index.php?option=com_content&view=article&id=90:kestrel-4500-anemc3b3metro&catid=26)