Fix JSON link

[alexkreidler]

The link to the VSCode JSON language server is no longer accurate.

It points to: https://github.com/Microsoft/vscode/tree/master/extensions/json

However, the real language server is implemented here: https://github.com/microsoft/vscode/tree/master/extensions/json-language-features/server and in the rest of the json-language-features extension.

[keegancsmith]

@alexkreidler do you mind creating a PR for this?

[jdorfman]

Thanks @alexkreidler