fix css error

app.py

@@ -99,7 +99,7 @@ def submit_question(question):
 css = """
       #col-container {max-width: 80%; margin-left: auto; margin-right: auto;}
       #arithmedics-img {width: 200px; display: block; margin-left: auto; margin-right: auto;}
-      #submitbtn {width: 50%, margin:auto}
+      #submitbtn {width: 50%; margin:auto}
       #chatbox {min-height: 400px; .scroll-container {position: absolute; top: 0; right:0;}}
       #header {text-align: center;}
       #total_tokens_str {text-align: right; font-size: 0.8em; color: #666;}