Tweak gh attestation doc strings - #2

The documented dnf install lines pin resolution to the gh-cli repo
with --repo gh-cli. That repo only ships the gh package, so on a
fresh Fedora install without git already present the install fails
with "nothing provides git needed by gh-2.87.3-1.x86_64 from gh-cli"
(cli#12808). Dropping --repo lets dnf pull git from the system repos
while still installing gh from gh-cli.

The maintainer suggested this in the issue thread after spotting
that other projects (docker, vagrant) use addrepo without the
restrictive --repo flag.

Closes cli#12808

Signed-off-by: Charlie Tonneslan <[email protected]>