Promtail docs: Fix docker stage code example (#7653)

The current, 2.6.x, documentation on [configuring promtail
_pipeline_stages_](https://grafana.com/docs/loki/latest/clients/promtail/configuration/#pipeline_stages)
contains a [docker
stage](https://grafana.com/docs/loki/latest/clients/promtail/configuration/#docker)
which wraps the json, labels, timestamp, and output stage. The unwrapped
configuration is shown as example. However, this example is outdated as
can be seen by the lack of `expressions`. I have stumbled across this as
I tried the example in my own config as a starter to modify to my own
needs. It took me some time to realize what `failed to make file target
manager: invalid json stage config: JMES expression is required` meant.
This PR updates the example to match the current code at
<a6df09bc97/clients/pkg/logentry/stages/extensions.go (L16-L42)>

Going back in history points to #576 as the origin of the divergence.

docs/sources/clients/promtail/configuration.md

@@ -431,9 +431,10 @@ The Docker stage is just a convenience wrapper for this definition:
 
 ```yaml
 - json:
-    output: log
-    stream: stream
-    timestamp: time
+    expressions:
+      output: log
+      stream: stream
+      timestamp: time
 - labels:
     stream:
 - timestamp: