docs: fix subscription response field path

[vuyua9]

What was changed

• Update the introductory subscription example to read the new-head block number from response["result"].
• Keep the example aligned with the formatted subscription response shape returned by process_subscriptions().

Why

process_subscriptions() yields formatted subscription responses with a top-level subscription and nested result. The example currently checks response["number"], which is not the shape users receive when they copy this snippet.

Validation

• git diff --check origin/main...HEAD
• changed-file conflict marker scan clean

Note: this PR is opened first so the follow-up commit can add the required newsfragment using the PR number, per newsfragments/README.md.