Bump Snippet version (#127688)

## Description

Bumps the version for snippets to the latest version.

## Related Issues
 - https://github.com/flutter/flutter/issues/127320

## Tests
 - No test changes needed.

dev/bots/docs.sh

@@ -25,7 +25,7 @@ function generate_docs() {
     # Install and activate the snippets tool, which resides in the
     # assets-for-api-docs repo:
     # https://github.com/flutter/assets-for-api-docs/tree/master/packages/snippets
-    "$DART" pub global activate snippets 0.3.0
+    "$DART" pub global activate snippets 0.3.1
 
     # This script generates a unified doc set, and creates
     # a custom index.html, placing everything into dev/docs/doc.