[shared_preferences] Fix confusing language in README (#8049)

One use of "class" instead of "plugin" to refer to the `SharedPreferences` class specifically slipped through review. This updates it to make it clear that it's talking about a specific class, not the plugin.

Also updates the `setPrefix:` doc comment to clarify that it only applies to that class, now that the addition of other classes to the API surface has made that ambiguous.

Fixes flutter/flutter#158404

packages/shared_preferences/shared_preferences/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.3.3
+
+* Clarifies scope of prefix handling in README.
+
 ## 2.3.2
 
 * Removes outdated testing information from README.

packages/shared_preferences/shared_preferences/README.md

@@ -160,7 +160,7 @@ A tool to make this process easier can be tracked here: https://github.com/flutt
 
 #### Adding, Removing, or changing prefixes on SharedPreferences
 
-By default, the `SharedPreferences` plugin will only read (and write) preferences
+By default, the `SharedPreferences` class will only read (and write) preferences
 that begin with the prefix `flutter.`. This is all handled internally by the plugin
 and does not require manually adding this prefix.
 

packages/shared_preferences/shared_preferences/lib/src/shared_preferences_legacy.dart

@@ -28,7 +28,8 @@ class SharedPreferences {
   static SharedPreferencesStorePlatform get _store =>
       SharedPreferencesStorePlatform.instance;
 
-  /// Sets the prefix that is attached to all keys for all shared preferences.
+  /// Sets the prefix that is attached to all keys for all shared preferences
+  /// read or written via this class.
   ///
   /// This changes the inputs when adding data to preferences as well as
   /// setting the filter that determines what data will be returned

packages/shared_preferences/shared_preferences/pubspec.yaml

@@ -3,7 +3,7 @@ description: Flutter plugin for reading and writing simple key-value pairs.
   Wraps NSUserDefaults on iOS and SharedPreferences on Android.
 repository: https://github.com/flutter/packages/tree/main/packages/shared_preferences/shared_preferences
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+shared_preferences%22
-version: 2.3.2
+version: 2.3.3
 
 environment:
   sdk: ^3.4.0