doc: nrf54: Fix typo on board target

[ubieda]

nrf54l15pdk/nrf54l51/cpuapp to nrf54l15pdk/nrf54l15/cpuapp.

[NordicBuilder]

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[NordicBuilder]

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[ubieda]

CC @carlescufi

[github-actions]

This pull request has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this pull request will automatically be closed in 14 days. Note, that you can always re-open a closed pull request at any time.

[ubieda]

Any chance this could get merged in?