doc(ios): update swift-tools-version to 5.9 in iOS Plugin Development Guide (#1479)

- Was changed in apache/cordova-ios#1611: `Since we require Xcode 15.0 or newer, we can safely set the swift-tools-version to 5.9.`

Author: GitToTheHub

www/docs/en/latest/guide/platforms/ios/plugin.md

@@ -253,7 +253,7 @@ Starting from Cordova-iOS 8 and greater, support for the Swift Package Manager (
 In the plugin's root directory, create a new file called `Package.swift` with the following content:
 
 ```swift
-// swift-tools-version:5.5
+// swift-tools-version:5.9
 
 import PackageDescription