fix invalid syntax error:

app.py

@@ -79,7 +79,7 @@ class PDFChatbot:
             return
 
         print("Database is ready. Retrieving relevant documents...")
-        retriever = self.state.db.as_ retriever()
+        retriever = self.state.db.as_retriever()
         llm = ChatGoogleGenerativeAI(model=LLM_MODEL, temperature=0.7, google_api_key=google_api_key)
 
         prompt_template = PromptTemplate(