Update gradle example to use `plugins` block.

[junghoon-vans]

As currently written in the example code for gradle, It is the legacy way to set up plugins.

If we were to change to code that uses the plugins block, it would be a bit more user-friendly,
nd it would be a simple change to make by reducing the number of lines in the code example.

AS-IS

buildscript {
	repositories {
		mavenLocal()
		mavenCentral()
	}
	dependencies {
		classpath("io.spring.javaformat:spring-javaformat-gradle-plugin:0.0.40")
	}
}

apply plugin: 'io.spring.javaformat'

TO-BE

plugins {
	 id 'io.spring.javaformat' version '0.0.40'
}

[wilkinsona]

Thanks for the suggestion. Until the plugin is published to Gradle's plugin portal (#309), this change as proposed will break people's builds as they won't be able to resolve the plugin. It could be addressed by also configuring the plugin repositories in settings.gradle to add mavenCentral(). I'm not totally convinced it's worth it though. WDYT, @philwebb?

[philwebb]

Given that the project is mainly for ourselves, I don't think we should make the change. We can add a note to #309 to update the docs if we get publishing working.