Documentation/x86: Fix incorrect references to zero-page.txt

The file zero-page.txt does not exit. Add links to zero-page.rst
instead.

 [ bp: Massage a bit. ]

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201002190623.7489-1-xypron.glpk@gmx.de