Update app.py

Modified the custom tool to make it calculate the body mass index given a weight and a height.

app.py

@@ -13,10 +13,11 @@ def my_custom_tool(arg1:str, arg2:int)-> str: #it's import to specify the return
     #Keep this format for the description / args / args description but feel free to modify the tool
     """A tool that does nothing yet 
     Args:
-        arg1: the first argument
-        arg2: the second argument
+        arg1: A float representing a weight in kilograms.
+        arg2: A float representing a height in meters.
     """
-    return "What magic will you build ?"
+    bmi = arg1 / arg2**2
+    return bmi
 
 @tool
 def get_current_time_in_timezone(timezone: str) -> str: