Hopefully fix docs build for 1.9.0rc2

Author: foosel

.github/workflows/docs.yaml

@@ -24,7 +24,7 @@ jobs:
             jinja: "3.0.3"
             mermaid_fix: "true"
           - version: "1.9.0rc2"
-            source: "heads"
+            source: "tags"
             python: "3.7"
             sphinx: "4.5.0"
             mermaid: "0.7.1"