Update default_realworld.py

[Yuki-11]

Removed the listing of burstsr_val in the sampler.IndexedBurst argument. The sampler.IndexedBurst class does not expect datasets to be passed as a list, but rather expects a single dataset as an argument. This update ensures compatibility with the expected input type in IndexedBurst.

Currently, without this fix, the val dataset will not be processed correctly, as sampler.IndexedBurst is not designed to handle a list of datasets.