docs: adds all counts to one sum

[dwydler]

If both http-01 and dns-01 challenge are used, the original command returns two values.

jq ".[] | .Certificates | length" /data/acme.json
11
1

Which results in an error occurring in query.

traefik-certs-dumper  | + '[' -e /data/acme.json ]
traefik-certs-dumper  | + jq '.[] | .Certificates | length' /data/acme.json
traefik-certs-dumper  | + '[' 11 1 '!=' 0 ]
traefik-certs-dumper  | sh: 1: unknown operand

Therefore, the command is extended so that only one value is passed.

traefik-certs-dumper  | + '[' -e /data/acme.json ]
traefik-certs-dumper  | + jq '.[] | .Certificates | length' /data/acme.json
traefik-certs-dumper  | + jq -s add
traefik-certs-dumper  | + '[' 12 '!=' 0 ]

[ldez]

Thank you 👍