add missing link

Author: stlankes

src/doc/rustc/src/SUMMARY.md

@@ -17,6 +17,7 @@
     - [aarch64-apple-ios-sim](platform-support/aarch64-apple-ios-sim.md)
     - [armv7-unknown-linux-uclibceabi](platform-support/armv7-unknown-linux-uclibceabi.md)
     - [armv7-unknown-linux-uclibceabihf](platform-support/armv7-unknown-linux-uclibceabihf.md)
+    - [aarch64-unknown-none-hermitkernel](platform-support/aarch64-unknown-none-hermitkernel.md)
     - [\*-kmc-solid_\*](platform-support/kmc-solid.md)
     - [*-unknown-openbsd](platform-support/openbsd.md)
     - [x86_64-unknown-none](platform-support/x86_64-unknown-none.md)