Fix string format for multiple files input in Elwin tab.

[adriazalvarez]

Description of work

This PR fixes a bug in the formatting of multiple files as input for the Elwin tab in the Data Manipulation interface.
On this release cycle, there's been a lot of maintenance task in indirect/inelastic tabs. One of these changed some of the QString types used in presenters to std::string types. The string manipulation for both formats is slightly different, there were two lines in the string formatting function that we didn't correct during development when we modified the type. Now is it fixed.

Fixes #36725 .

Further detail of work

To test:

1. Open Data Manipulation -> Elwin tab interface.
2. On file input, click on Browse and multi-select files: irs26173_graphite002_red.nxs and irs2617_graphite002_red.nxs.
3. Click on Run.
4. Check the ADS, the Output workspace names should look like this:

Instead of this:

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[robertapplin]

Building this at the moment. Just want to check, does it need a release note?

[adriazalvarez]

I think not, since this was a regression.

[robertapplin]

This is a nice and clean fix, happy to approve 👍