docs: fix variable name

Description

It looks like the name of the variable that toggles on shared ES was changed at some point, but we forgot to update the documentation.

Supporting information

See that the variable in action here:

• https://gitlab.com/opencraft/dev/grove/-/blob/c7c845fb928c139298dbd5764ff9b687760bf75d/tools-container/grove-cli/grove/terraform.py#L73
• https://gitlab.com/search?search=GROVE_ENABLE_SHARED_ELASTICSEARCH&nav_source=navbar&project_id=35974502&group_id=1712298&search_code=true&repository_ref=main

Testing instructions

Verify that the new variable name is correct and that the repo does not contain any more occurrences to the old variable name.