docs: fix link to Promtail documentation (#4293)

* chore: fix link to Promtail documentation

The current link to Promtail installation documentation in the file leads to 404 error. I observed that the `../docs/clients/` folder was refactored into `../docs/sources/clients` at same time in past. However, the `loki/docs/sources/clients/promtail/installation.md` file is not very convenient if we talk about UX. So, I've replaced the wrong link with the one to rendered beautiful documentation

* chore: make link relative

production/README.md

@@ -8,7 +8,7 @@ Currently there are five ways to try out Loki, in order from easier to hardest:
 - [Build Loki from source](#build-and-run-from-source)
 - [Get inspired by our production setup](#get-inspired-by-our-production-setup)
 
-For the various ways to run `promtail`, the tailing agent, see our [Promtail documentation](../docs/clients/promtail/README.md).
+For the various ways to run `promtail`, the tailing agent, see our [Promtail documentation](../docs/sources/clients/promtail/installation.md).
 
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