hide bf chars more in sentences.json

in sentences.json, remove 
```
"+-<>[].,abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ+/=",
```
and hide bf chars in other sentences. do not forget that the characters the flag logic is dealing in checker.py must also occur as single chars in the sentences. That includes currently: "A", "FAKE", "/", "=", "PADING"