docs/devel: Correct uefi-vars-x64 device name

docs/devel: Correct uefi-vars-x64 device name

The documentation for UEFI variable storage in uefi-vars.rst
incorrectly listed the device name as `uefi-vars-x86`.

The correct device name as implemented in the source code is
`uefi-vars-x64`.

This commit updates the documentation to use the correct name,
aligning it with the implementation.

Signed-off-by: Nana Liu <nanliu@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit f65918497cc6b9034ce8f81a4df1d6407e110367)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>