Fix URL to docs from README.md (#1501)

Docs are now published, but to a slightly different URL.

Author: daveyc123

README.md

@@ -12,7 +12,7 @@ This extension adds language support for Swift to Visual Studio Code, providing
 
 # Documentation
 
-The official documentation for this extension is available at [vscode-swift](https://www.swift.org/vscode/documentation/vscode)
+The official documentation for this extension is available at [vscode-swift](https://docs.swift.org/vscode/documentation/userdocs)
 
 This extension uses [SourceKit LSP](https://github.com/apple/sourcekit-lsp) for the [language server](https://microsoft.github.io/language-server-protocol/overviews/lsp/overview/), which powers code completion. It also has a dependency on [LLDB DAP](https://marketplace.visualstudio.com/items?itemName=llvm-vs-code-extensions.lldb-dap) for debugging.