Update clearcache.asciidoc

Changed `field_data` to `fielddata` to be consistent with the indices stats output

Closes #2888

docs/reference/indices/clearcache.asciidoc

@@ -10,7 +10,7 @@ $ curl -XPOST 'http://localhost:9200/twitter/_cache/clear'
 --------------------------------------------------
 
 The API, by default, will clear all caches. Specific caches can be cleaned
-explicitly by setting `filter`, `field_data`, `query_cache` coming[1.4.0],
+explicitly by setting `filter`, `fielddata`, `query_cache` coming[1.4.0],
 or `id_cache` to `true`.
 
 All caches relating to a specific field(s) can also be cleared by