Correct --language to --languages in README

Author: hugovk

README.md

@@ -17,7 +17,7 @@ python3 ./build_docs.py --quick --build-root ./build_root --www-root ./www --log
 ```
 
 If you don't need to build all translations of all branches, add
-`--language en --branches main`.
+`--languages en --branches main`.
 
 
 # Check current version