RawText: update the show_text key of the hoverEvent JSON for 1.16

[AmauryCarrade]

On Minecraft 1.16+, the show_text hover action was changed from:

["",{"text":"Toast","color":"yellow","hoverEvent":{"action":"show_text","value":{"text":"Toasted!"}}}]

to:

["",{"text":"Toast","color":"yellow","hoverEvent":{"action":"show_text","contents":{"text":"Toasted!"}}}]

(the value key is now contents).

This should only be changed for 1.16+, as 1.15 will not understand that new format.