Fix typo in URI service extraction (introduced in 93624b0)

Also add a comment about lack of support in GDAL up to 2.2.x for
service specification in datasource uri

Ref #16626

python/plugins/db_manager/db_plugins/postgis/plugin.py

@@ -303,7 +303,8 @@ def info(self):
     def gdalUri(self, uri=None):
         if not uri:
             uri = self.database().uri()
-        service = (u'service=%s' % self.service()) if self.service() else ''
+        # NOTE: service-only URIs won't work with GDAL up to 2.2.x
+        service = (u'service=%s' % uri.service()) if uri.service() else ''
         schema = (u'schema=%s' % self.schemaName()) if self.schemaName() else ''
         dbname = (u'dbname=%s' % uri.database()) if uri.database() else ''
         host = (u'host=%s' % uri.host()) if uri.host() else ''