fix: missing margin-bottom on heading in "Working on Your Issue" section

### Description On the [Layer5 Community page](https://layer5.io/community/handbook/contributing), the "Working on Your Issue" section heading has no margin-bottom, causing the first list item to sit immediately below the heading with no spacing, making it cramped and hard to read. ### Screenshots <img width="1562" height="611" alt="Image" src="https://github.com/user-attachments/assets/c51591e9-df17-475f-9e7c-7c405c3ec948" /> ### Expected Behavior There should be proper spacing between the "Working on Your Issue" heading and the list items below it, consistent with other sections on the page. ### Environment: Host OS: Windows / macOS / Linux Browser: Chrome / Firefox / Safari The layer5.io website uses Gatsby, React, and GitHub Pages. Site content is found under the [`master` branch](https://github.com/layer5io/layer5/tree/master). - 📚 See [contributing instructions](https://github.com/layer5io/layer5/blob/master/CONTRIBUTING.md). - 🎨 Wireframes and [designs for Layer5 site](https://www.figma.com/file/5ZwEkSJwUPitURD59YHMEN/Layer5-Designs) in Figma [(open invite)](https://www.figma.com/team_invite/redeem/GvB8SudhEOoq3JOvoLaoMs) - 🙋🏾🙋🏼 Questions: [Discussion Forum](https://discuss.layer5.io) and [Community Slack](https://slack.layer5.io). <img src="https://raw.githubusercontent.com/layer5io/layer5/master/.github/assets/images/buttons/community.webp" height="22px" align="left" />Join the Layer5 Community by submitting your [community member form](https://layer5.io/newcomer).