Update gate clone error message for Zuul v3

Change-Id: I351de28aa5d5f25187953f1a4502445bc2ce76cc
diff --git a/functions-common b/functions-common
index deebdec..c0fbcaf 100644
--- a/functions-common
+++ b/functions-common
@@ -519,7 +519,7 @@
         if [[ ! -d $git_dest ]]; then
             if [[ "$ERROR_ON_CLONE" = "True" ]]; then
                 echo "The $git_dest project was not found; if this is a gate job, add"
-                echo "the project to the \$PROJECTS variable in the job definition."
+                echo "the project to 'required-projects' in the job definition."
                 die $LINENO "Cloning not allowed in this configuration"
             fi
             git_timed clone $git_clone_flags $git_remote $git_dest