fixed some typo in ex15.28.29/main.cpp

ch15/ex15.28.29/main.cpp

@@ -20,10 +20,10 @@
 //  Thus, the virtual net_price functions called are Quote::net_price. As a result,
 //  no discount was applied. The outcome was 9090
 //
-//  The obejcts held for this exercise are smart pointers to the Quote objects.In this
-//  case, ppolymorphism happened as expected.The actual virtual functions being called
+//  The objects held for this exercise are smart pointers to the Quote objects.In this
+//  case, polymorphism happened as expected.The actual virtual functions being called
 //  are Bulk_quote::net_price that ensure discount is applied.Thus, the outcome is 6363.
-//  It can be found that 30% dicount has been applied to the price calculation.
+//  It can be found that 30% discount has been applied to the price calculation.
 //
 //
 // If there is no discrepancy, explain why there isn’t one.