Use relative link to ehrQL

This keeps the link on the same site, if the site is somewhere other
than `docs.opensafely.org`; for example, when previewing the site.

docs/actions-scripts.md

@@ -34,7 +34,7 @@ Scripted actions can read and write output files that are saved in the workspace
 
 Outputs labelled `highly_sensitive` will not be visible to researchers. This is a [deliberate design feature of OpenSAFELY](https://www.opensafely.org/about/), intended to reduce the risk of disclosure of sensitive information. Outputs should **always** be classed as `highly_sensitive` if they are:
 
-- Pseudonymised patient-level outputs derived from queries run against Level 1 and 2 data, i.e., a specific study dataset generated by a [dataset_definition](https://docs.opensafely.org/ehrql/).
+- Pseudonymised patient-level outputs derived from queries run against Level 1 and 2 data, i.e., a specific study dataset generated by a [dataset definition](ehrql/index.md).
 - Pseudonymised patient-level intermediate outputs for a study derived from queries run against Level 3 data which output pseudonymised patient-level data i.e., a processed study dataset with certain filters/formatting applied.
 
 These types of outputs are considered potentially highly-disclosive, should not be pushed to Level 4, and are never intended for publishing outside the secure environment.