docs: Update sampling.md (#10987)

**What this PR does / why we need it**:

Fix parsing error in the doc


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docs/sources/send-data/promtail/stages/sampling.md

@@ -47,7 +47,7 @@ pipeline_stages:
       app:
 - match:
     pipeline_name: "app2"
-    selector: "{app="poki"}"
+    selector: '{app="poki"}'
     stages:
     - sampling:
         rate: 0.1