fixed broken link in faq section (#1825)

The link now redirects to main readme of the project as expected.

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

docs/project/faq.md

@@ -140,8 +140,8 @@ one.
 
 ## Why build Carbon?
 
-See the [project README](#why-build-carbon) for an overview of the motivation
-for Carbon. This section dives into specific questions in that space.
+See the [project README](/README.md#why-build-carbon) for an overview of the
+motivation for Carbon. This section dives into specific questions in that space.
 
 ### Why is performance critical?