Fix name of config option in comment

notebook/static/notebook/js/notebook.js

@@ -2741,7 +2741,8 @@ define([
                     var last_modified_check_margin = (that.config.data['last_modified_check_margin'] || 0.5) * 1000; // 500 ms
                     // We want to check last_modified (disk) > that.last_modified (our last save)
                     // In some cases the filesystem reports an inconsistent time,
-                    // so we allow 0.5 seconds (configurable in nbconfig/notebook.json as `last_modified_buffer`) difference before complaining.
+                    // so we allow 0.5 seconds difference before complaining.
+                    // This is configurable in nbconfig/notebook.json as `last_modified_check_margin`.
                     if ((last_modified.getTime() - that.last_modified.getTime()) > last_modified_check_margin) {  
                         console.warn("Last saving was done on `"+that.last_modified+"`("+that._last_modified+"), "+
                                     "while the current file seem to have been saved on `"+data.last_modified+"`");