Use tsx instead of typescriptx as formatting option (SharePoint#1724)

It seems typescriptx isn't supported in many markdown formatters

Author: mcraiha

docs/spfx/web-parts/guidance/build-custom-property-pane-controls.md

@@ -254,7 +254,7 @@ When creating a custom property pane control that uses React in the SharePoint F
 
 4. Define the asynchronous dropdown React component. In the **src/controls/PropertyPaneAsyncDropdown/components** folder, create a new file named **AsyncDropdown.tsx**, and enter the following code:
 
-  ```typescriptx
+  ```tsx
   import * as React from 'react';
   import { Dropdown, IDropdownOption } from 'office-ui-fabric-react/lib/components/Dropdown';
   import { Spinner } from 'office-ui-fabric-react/lib/components/Spinner';

docs/spfx/web-parts/guidance/tutorial-share-data-between-web-parts-global-variable.md

@@ -502,7 +502,7 @@ The Recent Document web part displays information about the most recently modifi
 
 2. In the code editor, open the **./src/webparts/recentDocument/components/RecentDocument.tsx** file, and paste the following code:
 
-  ```typescriptx
+  ```tsx
   import * as React from 'react';
   import {
     DocumentCard,