Fix challenge 15 (SIMD) link and title (#240)

The move from 14 to 15 wasn't entirely complete.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Author: tautschnig

doc/src/SUMMARY.md

@@ -26,4 +26,4 @@
   - [12: Safety of `NonZero`](./challenges/0012-nonzero.md)
   - [13: Safety of `CStr`](./challenges/0013-cstr.md)
   - [14: Safety of Primitive Conversions](./challenges/0014-convert-num.md)
-  - [15: Contracts and Tests for SIMD Intrinsics](./challenges/0014-intrinsics-simd.md)
+  - [15: Contracts and Tests for SIMD Intrinsics](./challenges/0015-intrinsics-simd.md)

doc/src/challenges/0015-intrinsics-simd.md

@@ -1,4 +1,4 @@
-# Challenge 14: Contracts and Tests for SIMD Intrinsics
+# Challenge 15: Contracts and Tests for SIMD Intrinsics
 
 - **Status:** Open
 - **Reward:**