Update CONTRIBUTING.md (#1776)

Fix broken link.

Author: sean-mcmanus

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 ## Contribution Steps
 
-* [Build and debug the extension](Documentation/Getting%20started.md#build-and-debug-the-cpptools-extension).
+* [Build and debug the extension](Documentation/Building%20the%20Extension.md).
 * File an [issue](https://github.com/Microsoft/vscode-cpptools/issues) and a [pull request](https://github.com/Microsoft/vscode-cpptools/pulls) with the change and we will review it.
 * If the change affects functionality, add a line describing the change to [**CHANGELOG.md**](Extension/CHANGELOG.md).
 * Try and add a test in [**test/extension.test.ts**](Extension/test/unitTests/extension.test.ts).