Fix the plugin dependency identifier in the docs

[lnhrdt]

Fixes the instructions for adding the Gradle plugin to the buildscript classpath in the user guide.

Based on the instructions in the Gradle plugin portal:
https://plugins.gradle.org/plugin/com.github.johnrengelman.shadow

Fixes #750.

[tajchert]

This is a good change thank you :) I didn't realize and it took me a while to find source of my problem

[LIMPIX31]

Nice

[lnhrdt]

Hey @johnrengelman! I'm upgrading to 8.1.0 finally and finding the configuration shown in the docs broken again. Not sure why it keeps changing but maybe you do!

Right now your docs suggest this to add the plugin to the buildscript classpath:

buildscript {
    repositories {
        gradlePluginPortal()
    }
    dependencies {
        classpath 'gradle.plugin.com.github.johnrengelman:shadow:8.1.0'
    }
}

but the correct configuration (found on Gradle's plugin site) suggest:

buildscript {
  repositories {
    maven {
      url = uri("https://plugins.gradle.org/m2/")
    }
  }
  dependencies {
    classpath("com.github.johnrengelman:shadow:8.1.0")
  }
}

Maybe just link to the Gradle site from your docs? Or maybe you know why your published artifact IDs have been changing?

[johnrengelman]

They’ve been changing because Gradles been changing how plugins are published (first with bintray going away and then some other internal stuff)
I need to publish a bit about the. Testing change in the recent version, life got in the way.
good thought on just linking out. (Most people should just use the plugin method instead of declaring the dependency directly)