docs(outputs.zabbix): JSON Lines output is fenced as json

[jstirnaman]

Summary

In plugins/outputs/zabbix/README.md, three output examples are fenced as
json. Each shows newline-delimited JSON objects (one per line), which isn't a
single JSON document.

{"host": "hostname", "key": "telegraf.measurement.valueA", "value": "0"}
{"host": "hostname", "key": "telegraf.measurement.valueB", "value": "1"}

Suggested fix

Fence the three blocks as jsonl.