Replace China with Mainland China
Context: https://forum.f-droid.org/t/topic/16063/26 https://hosted.weblate.org/translate/f-droid/website/zh_Hans/?checksum=a1e46e4ac5c3396b#comments
Some users from Mainland China don't want to display Taiwan and China side by side. This MR use Taiwan and Mainland China instead.
Edited by Hans-Christoph Steiner