Update mapper-size.asciidoc

Small change in wording.

docs/plugins/mapper-size.asciidoc

@@ -26,7 +26,7 @@ PUT my-index-000001
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 The value of the `_size` field is accessible in queries, aggregations, scripts,
-and when sorting. It can be retrieved using the {ref}/search-fields.html#search-fields-param[fields API]:
+and when sorting. It can be retrieved using the {ref}/search-fields.html#search-fields-param[fields parameter]:
 
 [source,console]
 --------------------------