Fix regex pipeline-example.yml (#1707)

Backslashes must not be escaped inside single quotes:
https://github.com/grafana/loki/blob/master/docs/clients/promtail/stages/regex.md#schema

Spent an hour trying to figure out why no `level` tags assigned to my logs. Worked fine after the fix.

cmd/docker-driver/pipeline-example.yaml

@@ -1,5 +1,5 @@
 pipeline_stages:
 - regex:
-     expression: '(level|lvl|severity)=(?P<level>\\w+)'
+     expression: '(level|lvl|severity)=(?P<level>\w+)'
 - labels:
-    level:
+    level: