Make wording in nested nav recipe device-agnostic.

[antfitch]

Description of what this PR is changing or adding, and why:

The recipe assumed that the developer is building an Android app.

• Updated the wording so as not to exclude iOS apps.
• Small change to intro to make it clear which pages were being created.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 2de6d69):

https://flutter-docs-prod--pr11775-ios-nested-nav-bsy5mrk5.web.app

[sfshaza2]

lgtm!