Add docstring

Author: somacdivad

ch06-functions-and-loops/5-challenge-track-your-investments.py

@@ -6,6 +6,7 @@
 
 
 def invest(amount, rate, years):
+    """Display year on year growth of an initial investment"""
     for year in range(1, years + 1):
         amount = amount * (1 + rate)
         print(f"year {year}: ${amount:,.2f}")