Fix Challenge 18 rendering in book (model-checking#337)

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Author: Carolyn Zech

doc/src/SUMMARY.md

@@ -31,4 +31,4 @@
   - [15: Contracts and Tests for SIMD Intrinsics](./challenges/0015-intrinsics-simd.md)
   - [16: Verify the safety of Iterator functions](./challenges/0016-iter.md)
   - [17: Verify the safety of slice functions](./challenges/0017-slice.md)
-  - [18: Verify the safety of slice iter functions](./challenges/0018-slice-iter-pt1.md)
+  - [18: Verify the safety of slice iter functions](./challenges/0018-slice-iter.md)