docs(api): format WatchSource type (#3037)

Author: Aimony

src/api/reactivity-core.md

@@ -384,9 +384,7 @@ Watches one or more reactive data sources and invokes a callback function when t
   type WatchSource<T> =
     | Ref<T> // ref
     | (() => T) // getter
-    | T extends object
-    ? T
-    : never // reactive object
+    | (T extends object ? T : never) // reactive object
 
   interface WatchOptions extends WatchEffectOptions {
     immediate?: boolean // default: false