commands: update usage & example of translate command
commands: update usage & example of translate command

the language_code argument also supports language name,
not just the ISO code of that language.

while this doesn't implements what's requested in #431,
it surely does make that feature unimportant.

Closes: https://github.com/TheBastionBot/Bastion/issues/431

snkrsnkampa/Bastion,snkrsnkampa/Bastion