docs: Actually configure reno to use the main branch
docs: Actually configure reno to use the main branch

I used the wrong setting [1]

[1] https://docs.openstack.org/reno/latest/user/usage.html#configuring-reno

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

getpatchwork/git-pw