docs: fix: Use three backticks for code blocks

[kaapstorm]

SUMMARY

Fixes a tiny issue in CONTRIBUTING.md: A code block was opened with four backticks, attempted to be closed with three backticks, and then a second code block was opened with three backticks and closed with four. The result was that the three backticks appeared inside the code block as code, along with a line of plain text.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

BEFORE

AFTER

TESTING INSTRUCTIONS

Rendered Markdown locally.

ADDITIONAL INFORMATION

Fixes #19330

• Has associated issue: docs: tiny: Closing backticks and plain text appear inside code block #19330
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[srinify]

Thanks for the catch & fix!