Developer Guide: Update Introduction to YAML

see photoprism/photoprism#3913

docs/developer-guide/technologies/yaml.md

@@ -78,7 +78,7 @@ services:
       MARIADB_PASSWORD: "compo$$e" # sets password to "compo$e"
 ```
 
-Values that contain a `$` are otherwise [interpreted as a variable](https://docs.docker.com/compose/compose-file/12-interpolation/#interpolation). In this case, both the `$VARIABLE` and the `${VARIABLE}` syntax are supported. For details, please refer to the [Docker Compose documentation](https://docs.docker.com/compose/compose-file/12-interpolation/#interpolation).
+Values that contain a `$` are otherwise [interpreted as a variable](https://docs.docker.com/compose/compose-file/12-interpolation/#interpolation). In this case, both the `$VARIABLE` and the `${VARIABLE}` syntax are supported. Further details on the use of variables can be found in the [file format reference](https://docs.docker.com/compose/compose-file/12-interpolation/#interpolation).
 
 ### True / False ###