Minor custom world dimension type variable parity.

In the custom world settings the only setting that I have found has a different naming scheme to everything else is infiniburn where the settings are 
"infiniburn": "minecraft:infiniburn_nether" and
"infiniburn": "minecraft:infiniburn_end"
rather than the_nether and the_end. Since everything else is described as the_nether or the_end it would be more intuitive to keep this trend.