Fix incorrect SwiftPM plugin migration step (#11029)

This was caught by @brianquinlan while migrating the `cupertino_http`
plugin.

Review URL:
https://flutter-docs-prod--pr11029-brian-feedback-9wffzaxo.web.app/packages-and-plugins/swift-package-manager/for-plugin-authors#how-to-add-swift-package-manager-support-to-an-existing-flutter-plugin

Part of:
dart-lang/http#1276 (comment)

## Presubmit checklist

- [x] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
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doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
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of 80 characters or fewer.

src/_includes/docs/swift-package-manager/migrate-objective-c-plugin.md

@@ -225,7 +225,8 @@ The example below uses `ios`, replace `ios` with `macos`/`darwin` as applicable.
     If you would like to use a custom `modulemap` with your Swift package,
     refer to [Swift Package Manager's documentation][].
 
-1. Move all remaining files from `ios/Classes` to `ios/Sources/plugin_name`.
+1. Move all remaining files from `ios/Classes` to
+   `ios/plugin_name/Sources/plugin_name`.
 
 1. The `ios/Assets`, `ios/Resources`, and `ios/Classes` directories should now
    be empty and can be deleted.

src/_includes/docs/swift-package-manager/migrate-swift-plugin.md

@@ -157,7 +157,7 @@ The example below uses `ios`, replace `ios` with `macos`/`darwin` as applicable.
    For more instructions, see
    [https://developer.apple.com/documentation/xcode/bundling-resources-with-a-swift-package](https://developer.apple.com/documentation/xcode/bundling-resources-with-a-swift-package).
 
-1. Move all files from `ios/Classes` to `ios/Sources/plugin_name`.
+1. Move all files from `ios/Classes` to `ios/plugin_name/Sources/plugin_name`.
 
 1. The `ios/Assets`, `ios/Resources`, and `ios/Classes` directories should now
    be empty and can be deleted.