repo: Update towncrier title format

Author: achimnol

pyproject.toml

@@ -2,10 +2,10 @@
 package = "ai.backend.client"
 filename = "CHANGELOG.md"
 directory = "changes/"
-title_format = "{version} ({project_date})"
+title_format = "## {version} ({project_date})"
 template = "changes/template.md"
 issue_format = "([#{issue}](https://github.com/lablup/backend.ai-client-py/issues/{issue}))"
-underlines = ["-", "", ""]
+underlines = ["", "", ""]
 
 [[tool.towncrier.type]]
     directory = "breaking"