Listing 6.11 fix compile error

[TenzT]

Since const-ness(aka the modifiers const ) on function bucket_type::find_entry_for and bucket_type::value_for makes data.begin() and data.end() return a const_iterator, instead of a const iterator to be return. sees std::vector<T,Allocator>::begin
you might come across

no viable conversion from returned value of type 'std::__list_const_iterator<std::pair<std::string, int>, void *>' to function return type 'bucket_iterator' (aka '__list_iterator<std::pair<std::string, int>, void *>')

Fix it easily by removing the const on line 23 and line 30 and then have fun again!