Fix configuration in VS code setup docs (rust-lang#1264)

Co-authored-by: Adrian Palacios <73246657+adpaco-aws@users.noreply.github.com>

docs/src/rustc-hacks.md

@@ -15,7 +15,7 @@ In order to get these features working for `rustc` crates, you can do the follow
 
 Add the following to the `rust-analyzer` extension settings in `settings.json`:
 ```json
-    "rust-analyzer.rustcSource": "discover",
+    "rust-analyzer.rustc.source": "discover",
     "rust-analyzer.workspace.symbol.search.scope": "workspace_and_dependencies",
 ```