docs: add editor to list of plugin properties

Author: stee-re

docs/plugin-api.md

@@ -20,7 +20,7 @@ OpenSCD core communicates the data necessary for editing SCL documents by settin
 
 ```typescript
 export default class MyPlugin extends HTMLElement {
-  editor: XMLEditor;
+  editor: Transactor<EditV2>;
   docs: Record<string, XMLDocument> = {}; // all loaded documents
   doc?: XMLDocument; // the document currently being edited
   docName?: string; // the current doc's name