[pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/executablebooks/mdformat: 0.7.19 → 0.7.21](hukkin/mdformat@0.7.19...0.7.21)

Author: pre-commit-ci[bot]

.pre-commit-config.yaml

@@ -53,7 +53,7 @@ repos:
     hooks:
       - id: yamllint
   - repo: https://github.com/executablebooks/mdformat
-    rev: 0.7.19
+    rev: 0.7.21
     hooks:
       - id: mdformat
         additional_dependencies: