docs: fix docs build failing

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (b32d0ac) 94.92% compared to head (43a4195) 94.92%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #527 +/- ## ======================================= Coverage 94.92% 94.92% ======================================= Files 27 27 Lines 3034 3034 ======================================= Hits 2880 2880 Misses 154 154 ``` | [Flag](https://app.codecov.io/gh/abess-team/abess/pull/527/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team) | Coverage Δ | | |---|---|---| | [Python](https://app.codecov.io/gh/abess-team/abess/pull/527/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team) | `97.74% <ø> (ø)` | | | [rpackage](https://app.codecov.io/gh/abess-team/abess/pull/527/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team) | `93.58% <ø> (ø)` | | 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=abess-team#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/abess-team/abess/pull/527?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team) | Coverage Δ | | |---|---|---| | [R-package/R/abess.R](https://app.codecov.io/gh/abess-team/abess/pull/527?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team#diff-Ui1wYWNrYWdlL1IvYWJlc3MuUg==) | `99.56% <ø> (ø)` | | | [R-package/R/abesspca.R](https://app.codecov.io/gh/abess-team/abess/pull/527?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team#diff-Ui1wYWNrYWdlL1IvYWJlc3NwY2EuUg==) | `87.81% <ø> (ø)` | | | [R-package/R/abessrpca.R](https://app.codecov.io/gh/abess-team/abess/pull/527?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team#diff-Ui1wYWNrYWdlL1IvYWJlc3NycGNhLlI=) | `100.00% <ø> (ø)` | |

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