fix(contributing): point bug-report link at bug_report.yml template

[drewferg11-hermes-bot]

What this changes

One line in CONTRIBUTING.md: the bug-report link target changes from ../../issues/new to ../../issues/new?template=bug_report.yml.

-- **Bugs:** Open a [GitHub issue](../../issues/new) with the board you're using,
+- **Bugs:** Open a [GitHub issue](../../issues/new?template=bug_report.yml) with the board you're using,
   what you expected, what actually happened, and (if relevant) the serial log.

Why

Because .github/ISSUE_TEMPLATE/config.yml sets blank_issues_enabled: false, the old /issues/new link landed on the generic template chooser instead of pre-selecting the Bug Report form. The contributing text already describes a bug report (board, expected, actual, serial log) and the bug_report.yml form has matching fields (Area, ESP32 board, Firmware version, Install method, Module count). Adding ?template=bug_report.yml skips the chooser and opens the form directly.

Verification

• The link resolves to https://github.com/DrewFerg11/Split-Flap-Display/issues/new?template=bug_report.yml from the repo root, where CONTRIBUTING.md lives.
• The on-disk template filename is bug_report.yml (verified), so GitHub recognizes the template= value.
• No other links in CONTRIBUTING.md are changed — the Discord link, feature-request reference, and README setup link are untouched.
• The relative-path form (../../issues/...) is preserved so the link works from both GitHub and the docs site, per the issue's Constraints.

Closes #33