[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

@@ -21,7 +21,7 @@ repos:
         types_or: [yaml]
 
   - repo: https://github.com/executablebooks/mdformat
-    rev: 0.7.19 # Use the ref you want to point at
+    rev: 0.7.21 # Use the ref you want to point at
     hooks:
       - id: mdformat
         # Optionally add plugins