Fix typo in BossHelper spawn message

[rehashedsalt]

Found the typo when scrolling through reddit and figured I'd contribute a fix.

Fair warning: it's an untested change because I don't have a workspace set up at the moment, but it's literally three characters in some string literals in a glorified print statement. I think we're probably in the clear.

[p1ut0nium-git]

Fixed in version 2.2.10 - https://www.curseforge.com/minecraft/mc-mods/rough-mobs-revamped/files/2885523