Fix the example of add qmp hello-world example.
Without ":", make will report error:
../qapi/misc.json:573:2: line should end with ':'

Signed-off-by: Zihao Chang <changzihao1@huawei.com>
Message-Id: <20201201143308.1626-1-changzihao1@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>