fix: add missing .md file extension

[dlachaume]

Content

This PR includes a fix to enable the publication of the blog post introducing the UTXO snapshot converter command in the Mithril client CLI.

Pre-submit checklist

• Branch
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)
  • Add ADR blog post or Dev ADR entry (if relevant)
  • No new TODOs introduced

[github-actions]

Test Results

    3 files  ±0     88 suites  ±0   14m 39s ⏱️ +58s
2 014 tests ±0  2 014 ✅ ±0  0 💤 ±0  0 ❌ ±0 
3 548 runs  ±0  3 548 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 8b14914. ± Comparison against base commit d4d3bb4.