dns_dynu: Fix egrep regexp

[azuwis]

Tested with busybox 1.28.3 in OpenWrt:

# echo '{abc}' | egrep -o "{[^{]*}"
egrep: bad regex '{[^{]*}': Repetition not preceded by valid expression
# echo '{abc}' | egrep -o "\{[^{]*\}"
{abc}

[Neilpang]

please show me the "$response" string. I need to test on other platforms.

Thanks.

[azuwis]

[{"text_data":"sW3M4berQTbaMbYIe1yPd6fHZc3JdcGaaTcUNGZbngg","id":1111111,"domain_id":9999999,"domain_name":"foo.dynu.com","node_name":"_acme-challenge","hostname":"_acme-challenge.foo.dynu.com","record_type":"TXT","ttl":90,"content":"sW3M4berQTbaMbYIe1yPd6fHZc3JdcGaaTcUNGZbngg","state":true},{"text_data":"-eWTN8b-hoDC773XWUwdGHcb-9mWiLcPsarsdK--etY","id":2222222,"domain_id":9999999,"domain_name":"foo.dynu.com","node_name":"_acme-challenge","hostname":"_acme-challenge.foo.dynu.com","record_type":"TXT","ttl":90,"content":"-eWTN8b-hoDC773XWUwdGHcb-9mWiLcPsarsdK--etY","state":true}]