fix: don't document onceWriteDoc (#25)

`indexer.onceWriteDoc` was removed in
8caaf6a (#21). This removes its
mention in the readme.

`git grep -i onceWriteDoc` shows no remaining references to this method
outside the changelog.

README.md

@@ -141,10 +141,6 @@ The name of the table for storing backlinks (used internally for indexing).
 
 Index an array of documents. Documents can be in any order. Documents must have an `id` property, a `version` property that is unique, and a `links` property which is an array of version ids for the documents parent(s).
 
-### indexer.onceWriteDoc(version, listener)
-
-Set a listener for a doc at a specific version. Useful for performing an action based on completion of indexing of a document.
-
 ### indexer.deleteAll()
 
 Delete all documents and backlinks. Useful if you want to reset the index.