docs(ci): stop recommending HEAD~1

In a single-commit repo, this would lead to this error:

```
Error: fatal: ambiguous argument 'HEAD~1..HEAD': unknown revision
or path not in the working tree.
```

Now that we have the `--last` flag, this should work for the case
in which we want just to analyze the last commit, not specify a
range of commits via `--from` and `--to`.

Closes conventional-changelog#3892

docs/guides/ci-setup.md

@@ -42,7 +42,7 @@ jobs:
 
       - name: Validate current commit (last commit) with commitlint
         if: github.event_name == 'push'
-        run: npx commitlint --from HEAD~1 --to HEAD --verbose
+        run: npx commitlint --last --verbose
 
       - name: Validate PR commits with commitlint
         if: github.event_name == 'pull_request'