Correct the tier listing of wasm32-wasip2

This target is tier 2, not tier 3, and I forgot to update this.

Closes rust-lang#133206

Author: alexcrichton

src/doc/rustc/src/platform-support/wasm32-wasip2.md

@@ -1,6 +1,6 @@
 # `wasm32-wasip2`
 
-**Tier: 3**
+**Tier: 2**
 
 The `wasm32-wasip2` target is a new and still (as of January 2024) an
 experimental target. This target is an extension to `wasm32-wasip1` target,