Update LlamaJoinCaravanEvent arguments

[PureGero]

caravanHead was renamed to tail in 26.1

Without the fix in this PR, this exception is thrown:

[21:13:07 ERROR]: Entity threw exception at world:197.0565253594869,71.0,352.56243817392556
java.lang.NullPointerException: Cannot invoke "net.minecraft.world.entity.animal.equine.Llama.getBukkitEntity()" because "this.caravanHead" is null
	at net.minecraft.world.entity.animal.equine.Llama.joinCaravan(Llama.java:492)

[granny]

Thank you!