Update app.py

Fixed bug (not describing the argument with correct name)

app.py

@@ -7,7 +7,7 @@ def search_wikipedia_pages(search_string: str) -> str:
     """This tool helps to search for wikipedia pages related to a search string. 
     The output are the first 10 pages related to the search string.
     Args:
-        arg1: the search string
+        search_string: the search string
     """
     try:
         page_titles = wikipedia.search(search_string, results=10)