Update doc/src/core-transmutation.md

Author: tautschnig

doc/src/core-transmutation.md

@@ -1,7 +1,7 @@
 # Challenge: Verify `core` transmuting methods
 
 - **Status:** Open
-- **Tracking Issue:** *Link to issue*
+- **Tracking Issue:** [Link to issue](https://github.com/model-checking/verify-rust-std/issues/19)
 - **Start date:** *YY/MM/DD*
 - **End date:** *YY/MM/DD*