fixed a little type in ex15.24.25.cpp

ch15/ex15.24.25.cpp

@@ -17,7 +17,7 @@
 // Why did we define a default constructor for Disc_quote? What effect, if
 // any, would removing that constructor have on the behavior of Bulk_quote?
 //
-//  Without it, when building the statement below, the compiler would conplain
+//  Without it, when building the statement below, the compiler would complain
 //  that:
 //
 // note: 'Bulk_quote::Bulk_quote()' is implicitly deleted because the default