Bump @antora/collector-extension in /docs

---
updated-dependencies:
- dependency-name: "@antora/collector-extension"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/package.json

@@ -2,7 +2,7 @@
   "dependencies": {
     "antora": "3.2.0-alpha.5",
     "@antora/atlas-extension": "1.0.0-alpha.2",
-    "@antora/collector-extension": "1.0.0-alpha.4",
+    "@antora/collector-extension": "1.0.0-alpha.6",
     "@asciidoctor/tabs": "1.0.0-beta.6",
     "@springio/antora-extensions": "1.12.0",
     "@springio/asciidoctor-extensions": "1.0.0-alpha.11"