Fix field name for spectra crossid queries

Author: bsipocz

astroquery/sdss/core.py

@@ -138,7 +138,7 @@ def query_crossid_async(self, coordinates, obj_names=None,
                 photobj_fields.append('p.objID as obj_id')
             else:
                 photobj_fields = []
-                specobj_fields.append('s.objID as obj_id')
+                specobj_fields.append('s.SpecObjID as obj_id')
 
         sql_query += ', '.join(photobj_fields + specobj_fields)