DOC: add encoding to json converter example

Without the encoding `log-format` will issue a warning like:

$ haproxy -c -f /etc/haproxy/haproxy.cfg
[WARNING] 073/180933 (179) : parsing [/etc/haproxy/haproxy.cfg:46] :
'log-format' : sample fetch <capture.req.hdr(1),json> failed with : missing
args for conv method 'json'
1 file changed