[Doc] Update Windows mingw build doc Qt URLs
[Doc] Update Windows mingw build doc Qt URLs

The URL for the referenced Qt download files have changed, updating to
new URLs.
NOTE: Version referenced is not changed, just the download URLs.

BTCfork/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,BitcoinUnlimited/BitcoinUnlimited,marlengit/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,marlengit/BitcoinUnlimited,Bitcoin-com/BUcash,Bitcoin-com/BUcash,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,cddjr/BitcoinUnlimited,cddjr/BitcoinUnlimited,Justaphf/BitcoinUnlimited,cddjr/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,Justaphf/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,marlengit/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,marlengit/BitcoinUnlimited,Justaphf/BitcoinUnlimited,cddjr/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Bitcoin-com/BUcash,marlengit/BitcoinUnlimited,Bitcoin-com/BUcash,krzysztofwos/BitcoinUnlimited,Bitcoin-com/BUcash,BitcoinUnlimited/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,Bitcoin-com/BUcash,BitcoinUnlimited/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu,marlengit/BitcoinUnlimited