documentation: dense_hash_map example uses std::tr1::hash<const char*>

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Problem with std::tr1::hash<const char*> that it casts pointer to size_t type 
instead of hashing the string. Even example may work as it is, any attempts to 
extend example to real application will fail.

Original issue reported on code.google.com by [e-mail] on 6 Nov 2012 at 8:49