Fix link rendering in 0014-convert-num.md (model-checking#293)

https://model-checking.github.io/verify-rust-std/challenges does not
autoconvert github issue links

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Co-authored-by: Michael Tautschnig <[email protected]>

Author: JasonGross

doc/src/challenges/0014-convert-num.md

@@ -1,7 +1,7 @@
 # Challenge 14: Safety of Primitive Conversions
 
 - **Status:** Resolved
-- **Tracking Issue:** https://github.com/model-checking/verify-rust-std/issues/220
+- **Tracking Issue:** [#220](https://github.com/model-checking/verify-rust-std/issues/220)
 - **Start date:** 2024/12/15
 - **End date:** 2025/2/28
 - **Prize:** *TBD*