Fix broken links in vscode namespace API

[mjclemente]

Here is where the current page can be viewed: https://code.visualstudio.com/docs/extensionAPI/vscode-api

Here's a list of the links on the page that were broken; they were all due to missing hashtags, so they didn't properly point to the anchors:

• https://code.visualstudio.com/docs/extensionAPI/TreeView.visible
• https://code.visualstudio.com/docs/extensionAPI/EventEmitter
• https://code.visualstudio.com/docs/extensionAPI/CodeAction
• https://code.visualstudio.com/docs/extensionAPI/registerDocumentFormattingEditProvider
• https://code.visualstudio.com/docs/extensionAPI/CompletionItem.additionalTextEdits

[jrieken]

@mjclemente Thanks! 👏