Fix tag  subs. input variables

Change-Id: I1733c775816057ecb16acbf573c0165c19445d0d
diff --git a/tag-git-repos.groovy b/tag-git-repos.groovy
index dabbb7f..312ec9e 100644
--- a/tag-git-repos.groovy
+++ b/tag-git-repos.groovy
@@ -44,7 +44,7 @@
         repoCommit = repoArray[2]
         if (repoCommit.contains('SUBS_SOURCE_REF')) {
           common.warningMsg("Replacing SUBS_SOURCE_REF => ${SOURCE_TAG}")
-          repoCommit.replace('SUBS_SOURCE_REF', SOURCE_TAG
+          repoCommit = repoCommit.replace('SUBS_SOURCE_REF', SOURCE_TAG
             )
         }
         gitRepoAddTag(repoUrl, repoName, TAG, GIT_CREDENTIALS, repoCommit)