Fix wrong groupId in docs

- Backport #852
- Fixes #880

Author: jvalkeal

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

@@ -29,7 +29,7 @@ With _maven_ you're expected to have something like:
         <artifactId>spring-boot-starter</artifactId>
     </dependency>
     <dependency>
-        <groupId>org.springframework.boot</groupId>
+        <groupId>org.springframework.shell</groupId>
         <artifactId>spring-shell-starter</artifactId>
     </dependency>
 </dependencies>