add branch auto-update in manifest.v2.schema.json

[Thovi98]

see https://ci-apps-dev.yunohost.org/ci/job/20709

[zamentur]

Could you explain what the goal of this change ?

[Thovi98]

remove the following warning:

    - Error validating manifest using schema: in key resources > sources > main_18_oca > autoupdate
       Additional properties are not allowed ('branch' was unexpected)
    - Error validating manifest using schema: in key resources > sources > main_17_oca > autoupdate
       Additional properties are not allowed ('branch' was unexpected)
    - Error validating manifest using schema: in key resources > sources > main_16_oca > autoupdate
       Additional properties are not allowed ('branch' was unexpected)

I may be wrong though :D