fix external_db_id definition in xref table
fix external_db_id definition in xref table

mjg17/ensembl,willmclaren/ensembl,at7/ensembl,at7/ensembl,at7/ensembl,muffato/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,willmclaren/ensembl,james-monkeyshines/ensembl,muffato/ensembl,Ensembl/ensembl,danstaines/ensembl,muffato/ensembl,muffato/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,danstaines/ensembl,at7/ensembl,mjg17/ensembl,Ensembl/ensembl