Fix option names in json parser docs

plugins/parsers/json/README.md

@@ -106,9 +106,9 @@ Config:
 ```toml
 [[inputs.file]]
   files = ["example"]
-  name_key = "name"
+  json_name_key = "name"
   tag_keys = ["my_tag_1"]
-  string_fields = ["b_my_field"]
+  json_string_fields = ["b_my_field"]
   data_format = "json"
 ```