update guide 8.5 to 8.6 has incorrect gitlab-shell dependency
I just updated my instance of gitlab from 8.5 to 8.6.
The update guide mentions a dependency to gitlab-shell 2.6.11.
However, during the installation and the final checks, a warning mentioning a dependency with gitlab-shell 2.6.12 is displayed.
The fix is simple: replace v2.6.11 by v2.6.12 in the update guide.