doc: Correct code block on Updating the Thingy:91 X firmware

[divipillai]

Correct the code to flash modem with Jlink on Thingy91:X and for Application firmware.
NCSDK-31070.

[NordicBuilder]

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Build number: 6

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[NordicBuilder]

You can find the documentation preview for this PR at this link. It will be updated about 10 minutes after the documentation build succeeds.

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[divipillai]

@gmarull : Could you please check the doc build error. This could be seen in some of the open PR. https://github.com/nrfconnect/sdk-nrf/actions/runs/12427923169/job/34698593602?pr=19656