Update reference link documentation in the adjust_sigmoid function in exposure.py

[jurneo]

update the broken link for the reference for exposure module

Description

minor update on the broken link for the reference paper in adjust_sigmoid in exposure module

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[mkcor]

Thank you!

[mkcor]

Merging now, since it's a tiny docs PR. The CI check that failed seems to have simply not run, and I don't see how it could relate to this PR.