Update reward amount for challenge 0010-string

Author: feliperodri

doc/src/challenges/0010-string.md

@@ -4,7 +4,6 @@
 - **Tracking Issue:** [#61](https://github.com/model-checking/verify-rust-std/issues/61)
 - **Start date:** *2024/08/19*
 - **End date:** *2025/04/10*
-- **Reward:** *N/A*
 - **Reward:** *10000 USD*
 
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