Fix stepdown command in double_rollback.js

The following stepdown command in double_rollback.js will trigger an error "stepdown period must be longer than secondaryCatchUpPeriodSecs", because default secondaryCatchUpPeriodSecs is 10 seconds, greater than the given 1 second stepdown period.

            assert.commandWorked(primary.adminCommand({replSetStepDown: 1}));