[Shimadzu] [CORRECT] Fix name of index group in output files

Description

Scientists discovered the corrected Shimadzu data couldn't be loaded with EXtra-data. extra-data-validate also failed. We realised that the index group was called index_group instead of data:

├INDEX
│ ├SPB_EHD_MIC
│ │ └CORR
│ │   └HPVX2_1:output
│ │     └index_group
│ │       ├count    [uint64: 40]
│ │       └first    [uint64: 40]

I'm not sure how we missed this before merging.

How Has This Been Tested?

In production. 😎 We have confirmed that the newly corrected data can be opened.

Relevant Documents (optional)

https://redmine.xfel.eu/issues/172510

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@schmidtp @ahmedk