Updated todo for maven-gpg-plugin in case of updating to ASF-18

git-svn-id: https://svn.apache.org/repos/asf/maven/pom/trunk@1711660 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/maven/pom.xml b/maven/pom.xml
index b78413f..580c007 100644
--- a/maven/pom.xml
+++ b/maven/pom.xml
@@ -953,7 +953,7 @@
           <artifactId>maven-toolchains-plugin</artifactId>
           <version>1.1</version>
         </plugin>
-        <!--remove when apache-parent updates this plugin -->
+        <!--remove when apache-parent is updated to ASF-18 -->
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-gpg-plugin</artifactId>