Fix typo in gp_get_export_creds_type()
Should be EXP_CREDS_TYPE_OPTION, not EXP_CTX_TYPE_OPTION.
Fixes: e155f81d84f7 ("Add helper to find options in rpc messages")
Signed-off-by: Andrew Elble <aweits@rit.edu>
Reviewed-by: Robbie Harwood <rharwood@redhat.com>