docs: fix language for portability criteria

This is to align language in the WASI phase process with all pre-existing WASI repos.

https://github.com/WebAssembly/WASI/blob/main/Contributing.md#2-feature-description-available-wasi-subgroup

README.md

@@ -10,7 +10,7 @@ Phase 2
 
 - Dave Bakker (@badeend)
 
-### Phase 4 Advancement Criteria
+### Portability Criteria
 
 - At least two independent production implementations.
 - Implementations available for at least Windows, Linux & MacOS.