Fix javadoc

Author: nik9000

server/src/main/java/org/elasticsearch/index/mapper/FieldMapper.java

@@ -302,7 +302,7 @@ public void parse(ParseContext context) throws IOException {
     public abstract ValueFetcher valueFetcher(SearchLookup lookup, @Nullable String format);
 
     /**
-     * Fetch values for a particular document.
+     * Reads values for documents in a leaf. 
      */
     @FunctionalInterface
     public interface ValueFetcher {