Add a missing doc link

Author: alexcrichton

src/doc/rustc/src/SUMMARY.md

@@ -16,6 +16,7 @@
     - [aarch64-apple-ios-sim](platform-support/aarch64-apple-ios-sim.md)
     - [\*-kmc-solid_\*](platform-support/kmc-solid.md)
     - [x86_64-unknown-none](platform-support/x86_64-unknown-none.md)
+    - [wasm64-unknown-unknown](platform-support/wasm64-unknown-unknown.md)
 - [Target Tier Policy](target-tier-policy.md)
 - [Targets](targets/index.md)
     - [Built-in Targets](targets/built-in.md)