Reference latest Pixi version in README (#83)

README.md

@@ -25,7 +25,7 @@ GitHub Action to set up the [pixi](https://github.com/prefix-dev/pixi) package m
 ```yml
 - uses: prefix-dev/setup-pixi@v0.5.1
   with:
-    pixi-version: v0.15.2
+    pixi-version: v0.16.0
     cache: true
     auth-host: prefix.dev
     auth-token: ${{ secrets.PREFIX_DEV_TOKEN }}