Fix json format parameter

[pzygielo]

Current doc:

$ curl -i https://corona-stats.online?json=true
HTTP/2 200 
content-type: text/html; charset=utf-8
...

(HTML follows)

with format parameter:

$ curl -i https://corona-stats.online?format=json
HTTP/2 200 
content-type: application/json; charset=utf-8
...

(JSON follows)

[vercel[bot]]

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[sagarkarira]

👍