qgis3: Add GitHub handle to maintainers

See: https://trac.macports.org/ticket/55906

gis/qgis3/Portfile

@@ -11,7 +11,7 @@ set commit          36bf093
 github.setup        qgis QGIS ${commit}
 version             2.99.20180124
 categories          gis
-maintainers         vince
+maintainers         {vince @Veence}
 description         QGIS 3 is a user-friendly GIS based on Qt 5
 long_description    QGIS is an open-source alternative to ArcGIS or other commercial GIS.\
                     It can be used as a standalone GIS, or also as frontend to other,\