[Qt] fix typo in bitcoingui.cpp
[Qt] fix typo in bitcoingui.cpp

availble -> available
dogecoin/dogecoin,patricklodder/dogecoin,patricklodder/dogecoin,dogecoin/dogecoin,patricklodder/dogecoin,dogecoin/dogecoin,dogecoin/dogecoin,patricklodder/dogecoin,dogecoin/dogecoin,patricklodder/dogecoin,dogecoin/dogecoin