Fix GH action branch name

Author: nkammah

.github/workflows/doc-preview.yml

@@ -14,7 +14,7 @@ jobs:
     if: github.event_name == 'pull_request_target'
     runs-on: ubuntu-latest
     steps:
-      - uses: elastic/docs/.github/actions/docs-preview@main
+      - uses: elastic/docs/.github/actions/docs-preview@master
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           repo: ${{ github.event.repository.name }}
@@ -24,7 +24,7 @@ jobs:
     if: github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     steps:
-      - uses: elastic/docs/.github/actions/docs-preview@main
+      - uses: elastic/docs/.github/actions/docs-preview@master
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           repo: ${{ github.event.repository.name }}