BUG/MINOR: stats: fix validity of the json schema

BUG/MINOR: stats: fix validity of the json schema

The json schema seems to be invalid when checking using the validator
from https://www.jsonschemavalidator.net/. Correct it using the
following specification :
http://json-schema.org/draft/2019-09/json-schema-validation.html#rfc.section.9.1

The impact of the bug it not well known as I am not sure of how useful
the json schema is for users. It is probably not used at all or else
this bug would have been reported.

This should be backported up to 1.8.

(cherry picked from commit a53ce4cc011250a0e561f4e5bb62f46719813534)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit a9299e1f56f65ce89d45690a2356d5f59eed6bdd)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>