fix environment variable name (#5451)

# Description
<!-- Please include a summary of the changes and the related issue.
Please also include relevant motivation and context. List any
dependencies that are required for this change. -->

Closes #<issue_number>

**Type of change**
<!-- Please delete options that are not relevant. Remember to title the
PR according to the type of change -->

- Bug fix (non-breaking change which fixes an issue)
- Documentation update

**How Has This Been Tested**
<!-- Please add some reference about how your feature has been tested.
-->

**Checklist**
<!-- Please go over the list and make sure you've taken everything into
account -->

- I added relevant documentation
- I followed the style guidelines of this project
- I did a self-review of my code
- I made corresponding changes to the documentation
- I confirm My changes generate no new warnings
- I have added tests that prove my fix is effective or that my feature
works
- I have added relevant notes to the CHANGELOG.md file (See
https://keepachangelog.com/)

Author: frascuchon

argilla/docs/reference/argilla-server/configuration.md

@@ -92,7 +92,7 @@ The following environment variables are useful only when PostgreSQL is used:
 
 ### Docker images only
 
-- `REINDEX_DATASET`: If `true` or `1`, the datasets will be reindexed in the search engine. This is needed when some search configuration changed or data must be refreshed (Default: `0`).
+- `REINDEX_DATASETS`: If `true` or `1`, the datasets will be reindexed in the search engine. This is needed when some search configuration changed or data must be refreshed (Default: `0`).
 
 - `USERNAME`: If provided, the owner username. This can be combined with HF OAuth to define the argilla server owner (Default: `""`).