add rawvec challenge

Author: thanhnguyen-aws

doc/src/challenges/0019-rawvec.md

@@ -1,7 +1,7 @@
 # Challenge 25: Verify the safety of `RawVec` functions
 
 - **Status:** Open
-- **Tracking Issue:** [#286](https://github.com/model-checking/verify-rust-std/issues/286)
+- **Tracking Issue:** [#283](https://github.com/model-checking/verify-rust-std/issues/283)
 - **Start date:** *2025-03-07*
 - **End date:** *2025-10-17*
 - **Reward:** *10000 USD*