Fix README documentation link

[JSap0914]

Summary

• Update the README documentation link to the current LiqPay docs page.

Source proof

• Fixes README documentation link returns 404 #13.
• Current README used https://www.liqpay.ua/documentation/en.
• Before: curl -L -s -o /dev/null -w returned HTTP 404 for https://www.liqpay.ua/documentation/en.
• Replacement target: curl -L -s -o /dev/null -w returned HTTP 200 for https://www.liqpay.ua/doc.

Verification

• git diff --check
• curl -L -s -o /dev/null -w status check for https://www.liqpay.ua/doc returned HTTP 200.

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