Fix name of Flutter class in plugin docstrings (#18889)

shell/platform/linux/fl_text_input_plugin.h

@@ -21,14 +21,14 @@ G_DECLARE_FINAL_TYPE(FlTextInputPlugin,
  * FlTextInputPlugin:
  *
  * #FlTextInputPlugin is a text_input channel that implements the shell side
- * of TextInputPlugins.textInput from the Flutter services library.
+ * of SystemChannels.textInput from the Flutter services library.
  */
 
 /**
  * fl_text_input_plugin_new:
  * @messenger: an #FlBinaryMessenger.
  *
- * Creates a new plugin that implements TextInputPlugins.textInput from the
+ * Creates a new plugin that implements SystemChannels.textInput from the
  * Flutter services library.
  *
  * Returns: a new #FlTextInputPlugin.