[release-2.9.x] docs: fix promtail configuration sample for compresse…

…d files (#10790)

Backport 93cd29f from #10784

---

**What this PR does / why we need it**:
Fixes a typo which references an unknown field in
[scrapeconfig.DecompressionConfig](https://github.com/grafana/loki/blob/main/clients/pkg/promtail/scrapeconfig/scrapeconfig.go#L56)


[Search](https://github.com/search?q=repo%3Agrafana%2Floki+initial_sleep&type=code
) for `initial_sleep` yields only this line in the docs, while
[search](https://github.com/search?q=repo%3Agrafana%2Floki+initial_delay&type=code)
for `initial_deplay` yields code matches.

Co-authored-by: Gergő Rubint <me@gergorubint.com>

docs/sources/send-data/promtail/_index.md

@@ -59,7 +59,7 @@ scrape_configs:
 - job_name: system
   decompression:
     enabled: true
-    initial_sleep: 10s
+    initial_delay: 10s
     format: gz
   static_configs:
   - targets: