Remove deprechiated pinocchio 3 branch link in Readme

[manumerous]

Hi, when checking out Pinocchio after a while I was confused about the statement that Pinocchio3 is only available in the preview branch with a dead link. So I thought it would be nice to quickly adapt this to make it clear for others.

[hrp2-14]

Hi ! This project doesn't usually accept pull requests on the main branch.
If this wasn't intentionnal, you can change the base branch of this PR to devel
(No need to close it for that). Best, a bot.

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
The possible extra labels are:

• build_collision (build Pinocchio with coal support)
• build_casadi (build Pinocchio with CasADi support)
• build_autodiff (build Pinocchio with CppAD support)
• build_codegen (build Pinocchio with CppADCodeGen support)
• build_extra (build Pinocchio with extra algorithms)
• build_mpfr (build Pinocchio with Boost.Multiprecision support)
• build_sdf (build Pinocchio with SDF parser)
• build_accelerate (build Pinocchio with APPLE Accelerate framework support)
• build_all (build Pinocchio with ALL the options stated above)

Thanks.
The Pinocchio development team.

[nim65s]

Thanks !

The base branch was changed.

[jcarpent]

@manumerous Thanks a lot for the nice update.
In fact, I've just realised that is already in the readme of the devel branch. See https://github.com/stack-of-tasks/pinocchio/tree/devel?tab=readme-ov-file#introducing-pinocchio-3.

I guess we can now close this PR.
We will proceed soon to a new release of Pinocchio.