fix typo in wdpa_clean() docs

[codecov[bot]]

Codecov Report

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

Project coverage is 95.16%. Comparing base (b29bd4d) to head (d1f2b69).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #83 +/- ## ======================================= Coverage 95.16% 95.16% ======================================= Files 10 10 Lines 600 600 ======================================= Hits 571 571 Misses 29 29 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.