message: Fix out of date info on UI package README
diff: This package provides common Azure UI elements for VS Code extensions:
  - [Telemetry and Error Handling](#telemetry-and-error-handling): Displays error messages and adds telemetry to commands/events.
  -- [AzureTreeDataProvider](#azure-tree-data-provider): Displays an Azure Explorer with Azure Subscriptions and child nodes of your implementation.
  -- [AzureBaseEditor](#azure-base-editor): Displays a text editor with upload support to Azure.
  +- [AzExtTreeDataProvider](#azure-extension-tree-data-provider): Displays an Azure Explorer with Azure Subscriptions and child nodes of your implementation.
  +- [AzExtTreeFileSystem](#azure-extension-tree-file-system): A virtual file system that supports viewing and editing single files in Azure.
  > NOTE: This package throws a `UserCancelledError` if the user cancels an operation. If you do not use `registerCommand`, you must handle this exception in your extension.
  @@ -56,14 +56,14 @@ registerEvent(
  Follow these steps to create your basic Azure Tree:
  -1. Create an `AzureTreeItem` (or `AzureParentTreeItem`) describing the items to be displayed under your subscription:
  +1. Create an `AzExtTreeItem` (or `AzExtParentTreeItem`) describing the items to be displayed under your subscription:
  ```typescript
  - export class WebAppTreeItem extends AzureTreeItem {
  + export class WebAppTreeItem extends AzExtTreeItem {
  public static contextValue: string = "azureWebApp";
  public readonly contextValue: string = WebAppTreeItem.contextValue;
  private readonly _site: Site;
  - constructor(parent: AzureParentTreeItem, site: Site) {
  + constructor(parent: AzExtParentTreeItem, site: Site) {
  super(parent);
  this._site = site;
  }
  @@ -108,7 +108,7 @@ Follow these steps to create your basic Azure Tree:
  1. Create an `AzureAccountTreeItemBase` that provides the subscriptions you just implemented. It must implement at least `createSubscriptionTreeItem`:
  ```typescript
  export class AzureAccountTreeItem extends AzureAccountTreeItemBase {
  - public createSubscriptionTreeItem(root: ISubscriptionRoot): SubscriptionTreeItemBase {
  + public createSubscriptionTreeItem(root: ISubscriptionContext): SubscriptionTreeItemBase {
  return new SubscriptionTreeItem(this, root);
  }
  }
  