[pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/adamchainz/blacken-docs: 78a9dcbecf4f755f65d1f3dec556bc249d723600 → dda8db18cfc68df532abf33b185ecd12d5b7b326](adamchainz/blacken-docs@78a9dcb...dda8db1)

Author: pre-commit-ci[bot]

.pre-commit-config.yaml

@@ -19,7 +19,7 @@ repos:
       - id: trailing-whitespace
 
   - repo: https://github.com/adamchainz/blacken-docs
-    rev: 78a9dcbecf4f755f65d1f3dec556bc249d723600 # frozen: 1.19.1
+    rev: dda8db18cfc68df532abf33b185ecd12d5b7b326  # frozen: 1.20.0
     hooks:
       - id: blacken-docs