Return values as const to make the compiler happy in typescript

Based on this thread:
microsoft/TypeScript#35423
I had to change the return type to be const to satisfy the typescript compiler

src/pages/useLocalStorage.md

@@ -127,6 +127,6 @@ function useLocalStorage<T>(key: string, initialValue: T) {
     }
   };
 
-  return [storedValue, setValue];
+  return [storedValue, setValue] as const;
 }
 ```