[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)

.pre-commit-config.yaml

@@ -18,7 +18,7 @@ repos:
       - id: end-of-file-fixer
       - id: mixed-line-ending
   - repo: https://github.com/executablebooks/mdformat
-    rev: 0.7.19
+    rev: 0.7.21
     hooks:
       - id: mdformat
   - repo: https://github.com/psf/black