Updated readme to reflect setInstance method

Author: mvanbeusekom

packages/in_app_purchase/in_app_purchase_platform_interface/README.md

@@ -12,7 +12,7 @@ To implement a new platform-specific implementation of `in_app_purchase`, extend
 [`InAppPurchasePlatform`][2] with an implementation that performs the
 platform-specific behavior, and when you register your plugin, set the default
 `InAppPurchasePlatform` by calling
-`InAppPurchasePlatform.instance = MyPlatformInAppPurchase()`.
+`InAppPurchasePlatform.setInstance(MyPlatformInAppPurchase())`.
 
 # Note on breaking changes
 

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart

@@ -21,7 +21,7 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
 
   static final Object _token = Object();
 
-  /// The default instance of [InAppPurchasePlatform] to use.
+  /// The instance of [InAppPurchasePlatform] to use.
   ///
   /// Defaults to `null`.
   static InAppPurchasePlatform? get instance => _instance;

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/types/purchase_param.dart

@@ -14,7 +14,7 @@ class PurchaseParam {
 
   /// The product to create payment for.
   ///
-  /// It has to match one of the valid [ProductDetails] objects that you get from [ProductDetailsResponse] after calling [InAppPurchaseConnection.queryProductDetails].
+  /// It has to match one of the valid [ProductDetails] objects that you get from [ProductDetailsResponse] after calling [InAppPurchasePlatform.queryProductDetails].
   final ProductDetails productDetails;
 
   /// An opaque id for the user's account that's unique to your app. (Optional)