Fix jvmArgs usage example

The  configuration within an execution is only applied when that
particular execution is invoked as part of the build. It is not
applied when the plugin is invoked on the command line (i.e.
mvn spring-boot:run).

This commit clarifies the situation so that users can safely copy/paste
those bits and get the expected behavior.

 Fixes gh-875

spring-boot-tools/spring-boot-maven-plugin/src/site/apt/examples/run-debug.apt.vm

@@ -21,18 +21,11 @@
         <groupId>${project.groupId}</groupId>
         <artifactId>${project.artifactId}</artifactId>
         <version>${project.version}</version>
-        <executions>
-          <execution>
-            <goals>
-              <goal>run</goal>
-            </goals>
-            <configuration>
-              <jvmArguments>
-                  -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005
-              </jvmArguments>
-            </configuration>
-          </execution>
-        </executions>
+        <configuration>
+          <jvmArguments>
+            -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005
+          </jvmArguments>
+        </configuration>
         ...
       </plugin>
       ...