fix typo in sponsor section of main page
fix typo in sponsor section of main page

artardif/foss4g2017-dev,AvidGeo/foss4g2017,AvidGeo/foss4g2017-dev,artardif/foss4g2017-dev,AvidGeo/foss4g2017,AvidGeo/foss4g2017,AvidGeo/foss4g2017-dev,artardif/foss4g2017-dev,AvidGeo/foss4g2017-dev