Add missing closing details tag in nuctl testing step so text below is not hidden

[varun-singh9786]

Motivation and context

• Motivation: To fix contribution guideline
• Context: All docs below the last collapsible sections are hidden due to missing tag

How has this been tested?

• Since this is documentation only change, it has been tested manually.

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage remained the same at 65.311% when pulling f256387 on varun-singh9786:contrib-doc-details-tag-fix into c78cbb8 on openvinotoolkit:develop.

[bsekachev]

Thank you so much