Update sync_docs.yaml variables names discourse_api_username => disco…

…urse-api-user(#501)

The discourse_api_username and discourse_api_key are the key names expected by the discourse-gatekeeper action.

The data-platform-workflows version of sync_docs obtains the action's parameters via workflow inputs.
These input keys just don't match the action's keys because I wanted to keep the naming style consistent with other dpw workflow calls. Let's use the DPW variables discourse-api-user and discourse-api-key instead of the action's.

.github/workflows/sync_docs.yaml

@@ -12,8 +12,8 @@ jobs:
     name: Sync docs from Discourse
     uses: canonical/data-platform-workflows/.github/workflows/[email protected]
     secrets:
-      discourse_api_username: ${{ secrets.DISCOURSE_API_USERNAME }}
-      discourse_api_key: ${{ secrets.DISCOURSE_API_KEY }}
+      discourse-api-user: ${{ secrets.DISCOURSE_API_USERNAME }}
+      discourse-api-key: ${{ secrets.DISCOURSE_API_KEY }}
     permissions:
       contents: write  # Needed to push branch & tag
       pull-requests: write  # Needed to create PR