README: Fix sample buildscript producing duplicate files in dev jars

[makamys]

The sample buildscript snippet in the readme makes the dev jars of mods contain two copies of every file. For reference, here's the standard snippet for generating dev jars:

task devJar(type: Jar) {
	classifier = 'dev'
	from sourceSets.main.output
}

artifacts {
	archives devJar
}

Removing the output.resourcesDir = output.classesDir line fixes this. Was there a reason for including it?