docs: modify docstring for introduction

Author: Jeonghyun Min

financial/get_annuity_due_future_value.py

@@ -13,7 +13,8 @@
 More info on: https://www.investopedia.com/retirement/calculating-present-and-future-value-of-annuities
 
 This function can help to understand how much you will receive
-if you make a regular deposit for saving with a fix period and interest rate.
+if you make a regular deposit at the beginning of each term
+for saving with a fix period and interest rate.
 """