fixed bugs in ReadingLab/Discussion-for-Cpp#1

ch07/README.md

@@ -300,7 +300,7 @@ Both are noting happened.
 ```cpp
 (a) Sales_data &combine(Sales_data); // ok
 (b) Sales_data &combine(Sales_data&); // [Error] no matching function for call to 'Sales_data::combine(std::string&)' (`std::string&` can not convert to `Sales_data` type.)  
-(c) Sales_data &combine(const Sales_data&) const; // [Error] assignment of member 'Sales_data::units_sold' in read-only object. (we cannot combine the other `Sales_data` object.)
+(c) Sales_data &combine(const Sales_data&) const; // The trailing const mark can't be put here, as it forbids any mutation on data members. This comflicts with combine's semantics.
 ```
 Some detailed explanation about problem (b) :It's wrong. Because `combine`’s parameter is  a non-const reference , we can't  pass a temporary to that parameter. If `combine`’s parameter is  a  reference to const , we can  pass a temporary to that parameter. Like this :`Sales_data &combine(const Sales_data&); `  Here we call the `Sales_data` `combine` member function with a string argument. This call is perfectly legal; the compiler automatically creates a `Sales_data` object from the given string. That newly generated (temporary) `Sales_data` is passed to `combine`.(Also you can read C++ Primer Page 295(English Edition))