Update app.py

fix indent error

app.py

@@ -37,7 +37,7 @@ def get_bookbrain()-> str:
                 f"Title: {title}\nAuthor: {author}\nTopics: {topics}\nSummary: {summary}"
             )
             
-    return "\n\n---\n\n".join(books_str_list) if books_str_list else "No books found."
+            return "\n\n---\n\n".join(books_str_list) if books_str_list else "No books found."
     except Exception as e:
         return f"Failed to fetch books from BookBrain due to: {str(e)}"