Fix documentation errors in fetch_sdss_specgals().

The doc string for `fetch_sdss_specgals()` had a number of minor errors.  In addition, I updated the SQL query to include a join on `AstromDR9` to get the corrected DR9 astrometry.  Note however that this updated query would not match the data in http://www.astro.washington.edu/users/ivezic/DMbook/data/SDSSspecgalsDR8.fit .