Modify reward

Author: Carolyn Zech

doc/src/challenges/0016-iter.md

@@ -4,7 +4,7 @@
 - **Tracking Issue:** [#280](https://github.com/model-checking/verify-rust-std/issues/280)
 - **Start date:** *2025-03-07*
 - **End date:** *2025-10-17*
-- **Reward:** *5000 USD*
+- **Reward:** *10,000 USD*
 
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