fix invalid argument in spell:group

[Zbizu]

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Steps to reproduce

1. set two groups to Lua spell, for example spell:group(1, 2)
2. set two cooldowns like spell:groupCooldown(2000, 22000)
3. observe cooldowns bar

expected: two groups have cooldowns
observed: secondary group is ignored despite the function arguments suggesting otherwise

[EPuncker]

thanks