tx: Update transifex slug for 0.21

Author: laanwj

.tx/config

@@ -1,7 +1,7 @@
 [main]
 host = https://www.transifex.com
 
-[bitcoin.qt-translation-020x]
+[bitcoin.qt-translation-021x]
 file_filter = src/qt/locale/bitcoin_<lang>.ts
 source_file = src/qt/locale/bitcoin_en.ts
 source_lang = en