fix path to codemirror-assets.html

Bug: Issue 9060
Change-Id: I7c3604ace0e013de8b5ef3e018299f6755e3dcf4

gr-editor/gr-editor.js

diff --git a/gr-editor/gr-editor.js b/gr-editor/gr-editor.js
index 388b92f..a7eff27 100644
--- a/gr-editor/gr-editor.js
+++ b/gr-editor/gr-editor.js
@@ -58,7 +58,7 @@
     },
 
     _importCodeMirror() {
-      const url = this.plugin.url('static/codemirror-assets.html');
+      const url = this.plugin.url('/static/codemirror-assets.html');
       return new Promise((resolve, reject) => {
         this.importHref(url, resolve, reject);
       });