Update actions.js

Clarified "delete-cell" action.

IPython/html/static/notebook/js/actions.js

@@ -255,6 +255,7 define(function(require){
             }
         },
         'delete-cell': {
+            help: 'delete selected cell',
             help_index : 'ej',
             handler : function (env) {
                 env.notebook.delete_cell();