fix Cargo.toml and config.toml keys

docs/modules/ROOT/pages/faq.adoc

@@ -142,10 +142,11 @@ Note that the git revision should match any other embassy patches or git depende
 * Make sure link:https://docs.rs/cortex-m/latest/cortex_m/peripheral/struct.SCB.html[flash cache] is enabled
 * build with `--release`
 * Set the following keys for the release profile in your `Cargo.toml`:
-    ** `opt-level=s`
-    ** `lto=fat`
-    ** `build-std=core`
-    ** `build-std-features=panic_immediate_abort`
+    ** `opt-level = "s"`
+    ** `lto = "fat"`
+* Set the following keys in the `[unstable]` section of your `.cargo/config.toml`
+    ** `build-std = ["core"]`
+    ** `build-std-features = ["panic_immediate_abort"]`
 * Enable feature `embassy-time/generic-queue`, disable feature `embassy-executor/integrated-timers`
 * When using `InterruptExecutor`:
     ** disable `executor-thread`