chore: fix error

docs/explanation/service-dependencies.md

@@ -6,6 +6,6 @@ Simply put, you can configure a list of other services in the `requires` section
 
 When Pebble starts a service, it also starts the services which that service depends on (configured with `requires`). Conversely, when stopping a service, Pebble also stops services which depend on that service.
 
-For the start order of the services, see [Service start order](./service-start-order.txt).
+For the start order of the services, see [Service start order](./service-start-order.md).
 
 For example, if service `nginx` requires `logger`, `pebble start nginx` will start both `nginx` and `logger` (in an undefined order). Running `pebble stop logger` will stop both `nginx` and `logger`; however, running `pebble stop nginx` will only stop `nginx` (`nginx` depends on `logger`, not the other way around).