Improve the wazuh-docker upgrade guide

[rauldpm]

@cladkins reported a documentation inconsistency with the Docker upgrade guide since the wazuh-docker repository is not being updated with the latest changes:

• https://documentation.wazuh.com/current/deployment-options/docker/upgrading-wazuh-docker.html#using-default-docker-compose-files

The proposal is adding a git fetch command before the git checkout command

This makes sense since two options can occur.

1. The user performs the actions from a newly cloned repository that already has the reference to the new tag
2. The user uses the same repository from which they did the initial deployment

In any case, we are assuming that the user is going to keep the local repository updated and at a minimum, we must put a note for it to be updated, since if not, the git checkout will produce an error