Apply suggestion from @deboragracio

Co-authored-by: deboragracio <63159404+deboragracio@users.noreply.github.com>

Author: MaximilianFT

HOWTO_01_Payment_csharp/README.MD

@@ -41,6 +41,6 @@ In many payment vendor apps, it is possible to add a tip to the payment amount.
   - 2 pay items
   - Pay item 1: The total paid amount with the receipt -> 12€
   - Pay item 2: The tip with a negative amount -> -2€
-  - sum of the 2 pay items is the original requested amount of 10€
+  - The sum of the 2 pay items is the original requested amount of 10€
 
 NOTE: The tip can be calculated by simply subtracting the requested amount from the paid amount (12-10 = 2€ tip) or alternatively from the 2nd pay item (-2 * -1 = 2€ tip).