Update Cidr.rst

[etshy]

Update netmaskMax type to match the type in the code (?int).

[javiereguiluz]

Good catch @etshy. Thanks and congrats on your first Symfony Docs contribution 🎉

[etshy]

Thanks @javiereguiluz
Been a while I wanted to do something on symfony or doctrine as I use them a lot and I saw this by coïncidence while searching int arguments on annotation/attributes for a feature I requested on rector so it was perfect opportunity to start.