Proofread gallery example for Radon transform.

[mkcor]

Description

This minor PR follows from #5269.

The citations have rendering issues; it's not 100% satisfying to resolve them by removing the second instance of each citation, but I thought it was fine even in substance (no need to repeat "Kaczmarz' method [3]_" the exact same way if it's written a few sentences before; it's actually better not to).

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.

[emmanuelle]

Thanks @mkcor. CI failure (dask-compatibility tests) seem unrelated.