Resolve #3448437 "Enable merging on one term"

modules/taxonomy_manager_merge/js/tree.js

@@ -9,7 +9,7 @@
       document.addEventListener('taxonomy_manager-tree-select', function (event) {
         let data = event.detail || {};
 
-        if (data.tree.getSelectedNodes().length < 2) {
+        if (data.tree.getSelectedNodes().length < 1) {
           // Disable the button if less the two terms are selected.
           document.getElementById('edit-merge').disabled = true;
         } else {