fix query for DR8

Author: weaverba137

astroquery/sdss/core.py

@@ -555,7 +555,7 @@ def get_spectra_async(self, coordinates=None, radius=2. * u.arcsec,
 
         if not matches:
             request_payload = self._args_to_payload(
-                specobj_fields=['instrument', 'run2d', 'plate',
+                specobj_fields=['run2d', 'plate',
                                 'mjd', 'fiberID'],
                 coordinates=coordinates, radius=radius, spectro=True,
                 plate=plate, mjd=mjd, fiberID=fiberID,