Fix invalid note in environment selections doc

What does this MR do?

Fix an invalid note to use CAUTION instead of WARNING.

See https://docs.gitlab.com/ee/development/documentation/styleguide.html#caution

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content