The bibliography entry in Rmd documents is not code

[mwouts]

Closes #1161

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (405617f) 97.61% compared to head (0db460d) 97.61%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1175 +/- ## ======================================= Coverage 97.61% 97.61% ======================================= Files 29 29 Lines 4451 4451 ======================================= Hits 4345 4345 Misses 106 106 ``` | [Flag](https://app.codecov.io/gh/mwouts/jupytext/pull/1175/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | Coverage Δ | | |---|---|---| | [external](https://app.codecov.io/gh/mwouts/jupytext/pull/1175/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `75.12% <100.00%> (ø)` | | | [functional](https://app.codecov.io/gh/mwouts/jupytext/pull/1175/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `88.54% <100.00%> (ø)` | | | [integration](https://app.codecov.io/gh/mwouts/jupytext/pull/1175/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `77.37% <100.00%> (ø)` | | | [unit](https://app.codecov.io/gh/mwouts/jupytext/pull/1175/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts#carryforward-flags-in-the-pull-request-comment) to find out more.

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