Fix Vec3 pool size reporting in crash reports. Fixes BUKKIT-3378

Crash reports for exceptions in the server tick loop include information on the size of the Vec3 pool, but this is broken in CraftBukkit because of an invalid reference to MinecraftServer worldServer:

> Vec3 Pool Size: ~~ERROR~~ NoSuchFieldError: worldServer

This line appears in the crash reports in at least BUKKIT-220 BUKKIT-578 BUKKIT-758 BUKKIT-2959 BUKKIT-3016 BUKKIT-2371 BUKKIT-3368 BUKKIT-2926.

This PR updates CrashReportVec3DPoolSize to refer to the correct field (worlds), fixing the crash report Vec3 info:

>  0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used

Leaky ticket: https://bukkit.atlassian.net/browse/BUKKIT-3378