docs: update editor.editable to editor.isEditable (#1941)

Author: Keithcat767

docs/content/docs/reference/editor/overview.mdx

@@ -10,10 +10,10 @@ The BlockNote editor API is a comprehensive set of functions and methods that al
 
 ## Editable
 
-The editor is editable by default, but you can make it read-only by setting the `editable` property to `false`.
+The editor is editable by default, but you can make it read-only by setting the `isEditable` property to `false`.
 
 ```ts
-editor.editable = false;
+editor.isEditable = false;
 ```
 
 ## Focus