Register to HologramManager new holograms

[giopalma]

Without this line of code, the file is created, but the hologram is not registered. So any modification of the hologram's file doesn't work because it says it's already existing.

I'm not sure if DecentHologramsAPI.get().getHologramManager() is the best way to access HologramManager from the DHAPI class. It's what I used in my plugin and with solution it works.