Update blog tutorial to use pathlib

[patriacaelum]

The codeblock imported pathlib.Path but it was unused, so the operation for app.root_path / "blog.db" would error since app.root_path is defined as a string.

• fixes Blog tutorial has typo for path #347

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.