Update docs/backend/indexing.md

Co-authored-by: Jens W. Klein <[email protected]>

Author: stevepiercy

docs/backend/indexing.md

@@ -56,7 +56,7 @@ In your `profiles/default/types/YOURTYPE.xml` add the behavior.
 ```
 
 Now you need to mark the fields you want to have in your SearchableText. 
-This can be done with the searchable directive.
+This can be done with the `searchable` directive.
 
 ```python
 from plone.app.dexterity import textindexer