Update warnings.md (#5693)

## What are you changing in this pull request and why?
Small clarification that values should be passed on as arrays.
Previously (dbt 1.7.x) this worked but from 1.8.x. this no longer seems
to be the case.

## Checklist
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---------

Co-authored-by: Matt Shaver <[email protected]>

website/docs/reference/global-configs/warnings.md

@@ -82,6 +82,7 @@ DBT_WARN_ERROR_OPTIONS='{"include": ["NoNodesForSelectionCriteria"]}' dbt run
 ...
 ```
 
+Values for `error`, `warn`, and/or `silence` should be passed on as arrays. For example, `dbt --warn-error-options '{"error": "all", "warn": ["NoNodesForSelectionCriteria"]}' run` not `dbt --warn-error-options '{"error": "all", "warn": "NoNodesForSelectionCriteria"}' run`.
 
 
 <File name='profiles.yml'>