Fix path to task-provider-sample's main

It's built to out/extension.js, so launching the extension currently produces an error. I guess that was caused by this change:

https://github.com/Microsoft/vscode-extension-samples/commit/704f351abdcd509279cef358afa720b1e6c6961f#diff-71ee4bf336efb1f169e66d21bbc6d508