Fixed typo in maven site documentation

Closes gh-15176

Author: rs017991

spring-boot-tools/spring-boot-maven-plugin/src/site/apt/examples/custom-layout.apt

@@ -34,7 +34,7 @@
         </executions>
         <dependencies>
           <dependency>
-            <groupId>com.example</groupid>
+            <groupId>com.example</groupId>
             <artifactId>custom-layout</artifactId>
             <version>0.0.1.BUILD-SNAPSHOT</version>
           </dependency>