docs(bigquery): use equality operator in Errors example (googleapis#5201

)

Co-authored-by: shollyman <shollyman@shollyman.dev>

bigquery/doc.go

@@ -307,7 +307,7 @@ These errors can be introspected for more information by using `xerrors.As` with
 
 	var e *googleapi.Error
 	if ok := xerrors.As(err, &e); ok {
-		  if e.Code = 409 { ... }
+		  if e.Code == 409 { ... }
     }
 
 In some cases, your client may received unstructured googleapi.Error error responses.  In such cases, it is likely that