[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

@@ -2,7 +2,7 @@
 repos:
   - repo: https://github.com/executablebooks/mdformat
     # Do this before other tools "fixing" the line endings
-    rev: 0.7.19
+    rev: 0.7.21
     hooks:
       - id: mdformat
         name: Format Markdown