Upgrade Gradle plugin’s ID so it’s suitable for the Plugin Portal

Gradle’s plugin portal requires each plugin’s ID to be in a namespace.
Our existing ID, spring-boot, does not meet this requirement. This
commit changes the plugin’s ID to org.springframework.boot.spring-boot.
Note that, as is recommended [1], the plugin’s ID does not include
“gradle”.

See gh-1567

[1] http://plugins.gradle.org/submit

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -182,7 +182,7 @@ the `spring-boot` plugin:
 			classpath("org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}")
 		}
 	}
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot.spring-boot'
 ----
 
 If you are using a milestone or snapshot release you will also need to add appropriate

spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -235,7 +235,7 @@ endif::release[]
 	}
 
 	apply plugin: 'java'
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot.spring-boot'
 
 	jar {
 		baseName = 'myproject'

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -170,7 +170,7 @@ managed by Spring Boot:
 	}
 
 	apply plugin: 'java'
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot.spring-boot'
 
 	repositories { jcenter() }
 	dependencies {