Fix spelling error in spanish locale (#4221)

lib/network/locales.js

@@ -39,7 +39,7 @@ exports['de_DE'] = exports['de'];
 exports['es'] = {
   edit: 'Editar',
   del: 'Eliminar selecci\u00f3n',
-  back: '\u00c1tras',
+  back: 'Atr\u00e1s',
   addNode: 'A\u00f1adir nodo',
   addEdge: 'A\u00f1adir arista',
   editNode: 'Editar nodo',