[executorch][serialization] Format program.fbs with consistent whitespace (#6809)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6680

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ghstack-source-id: 253177479
@exported-using-ghexport

Differential Revision: [D65495297](https://our.internmc.facebook.com/intern/diff/D65495297/)

Co-authored-by: lucylq <lfq@meta.com>