Fixed typo in doc about weighted statistics

[schlunma]

Description

weighted -> weights

Link to documentation: https://esmvaltool--2387.org.readthedocs.build/projects/ESMValCore/en/2387/recipe/preprocessor.html#examples

---

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🧪 The new functionality is relevant and scientifically sound
• [x] 🛠 This pull request has a descriptive title and labels
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🧪 and 🛠 Documentation is available
• [x] 🛠 Unit tests have been added
• [x] 🛠 Changes are backward compatible
• [x] 🛠 Any changed dependencies have been added or removed correctly
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov[bot]]

Codecov Report

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

Project coverage is 94.07%. Comparing base (46207f8) to head (75ac62f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2387 +/- ## ======================================= Coverage 94.07% 94.07% ======================================= Files 241 241 Lines 13422 13422 ======================================= Hits 12627 12627 Misses 795 795 ```

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