Set the revision for the rstcheck pre-commit hook.

The pre-commit warnings has been annoying enough. The last tag in that
repo is pretty old (2018) and doesn't have the pre-commit hook, so let's
go with the last commit which happens to be adding the pre-commit hook.
Happy coincident.

.pre-commit-config.yaml

@@ -40,7 +40,7 @@ repos:
       - id: rst-linter
 
   - repo: https://github.com/myint/rstcheck.git
-    rev: master
+    rev: 3f92957478422df87bd730abde66f089cc1ee19b
     hooks:
       - id: rstcheck