Fix rest-api-spec dep for external plugins (#45949)

This commit fixes the maven coordinates for the rest-api-spec jar. It
was accidentally by #45107.

closes #45891

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/RestIntegTestTask.groovy

@@ -234,7 +234,7 @@ class RestIntegTestTask extends DefaultTask {
         }
         project.dependencies {
             restSpec ClasspathUtils.isElasticsearchProject() ? project.project(':rest-api-spec') :
-                    "org.elasticsearch.rest-api-spec:${VersionProperties.elasticsearch}"
+                    "org.elasticsearch:rest-api-spec:${VersionProperties.elasticsearch}"
         }
         Task copyRestSpec = project.tasks.findByName('copyRestSpec')
         if (copyRestSpec != null) {