chore: docs: update Epinio Swagger URL with version 1.13.8

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Author: epinio-bot

docs/references/api.md

@@ -17,7 +17,7 @@ curl -u your-username-here:password-here https://epinio.your-system-domain/api/v
 
 The responses are all JSON objects, except for some cases where you get an error before the request reaches the Epinio server (e.g. because of bad cluster configuration).
 
-The endpoints are documented in the [OpenAPI Spec](https://www.openapis.org/) format. You can find the definition of the API here: https://github.com/epinio/epinio/blob/v1.13.7/docs/references/api/swagger.json
+The endpoints are documented in the [OpenAPI Spec](https://www.openapis.org/) format. You can find the definition of the API here: https://github.com/epinio/epinio/blob/v1.13.8/docs/references/api/swagger.json
 
 __Make sure__ you change to the Epinio release you have deployed.