[google_maps_flutter_platform_interface] Fix PinConfig code sample

[meliharik]

The second PinConfig sample in the BitmapDescriptor docs (bitmap.dart:1009) doesn't compile, for two reasons:

1. Unbalanced parens — the block opens PinConfig(, BitmapGlyph( and BitmapDescriptor.asset( but only closes two of them. (The sibling TextGlyph sample directly above closes correctly.)
2. Missing await — BitmapDescriptor.asset is declared static Future<AssetMapBitmap> asset(...) (line 278), while BitmapGlyph.bitmap is final BitmapDescriptor bitmap; (line 1106), so a bare BitmapDescriptor.asset(...) can't be assigned to it.

The repo's own examples use the awaited form — e.g. google_maps_flutter_web/example/latest/integration_test/advanced_markers_test.dart:422:

bitmap: await BitmapDescriptor.asset(

The sample now mirrors that. Paren balance goes from 5 open / 4 close to 5 / 5.

Doc comment only; bumped to 2.16.1 with a CHANGELOG entry per the versioning policy — happy to switch to ## NEXT if that's preferred for a docs-only change.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran the auto-formatter.
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets.
• I [linked to at least one issue that this PR fixes] in the description above. (No tracker issue exists for this; it's a two-line doc-sample fix. Happy to file one in flutter/flutter if required.)
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or I have commented below to indicate which [version change exemption] this PR falls under.
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or I have commented below to indicate which [CHANGELOG exemption] this PR falls under.

[gemini-code-assist]

Code Review

This pull request updates the google_maps_flutter_platform_interface package to version 2.16.1. The changes correct the PinConfig code sample in the BitmapDescriptor documentation by adding the await keyword to the asynchronous BitmapDescriptor.asset call and adjusting the formatting. There are no review comments, and I have no feedback to provide.