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

Author: xypron

Documentation/x86/boot.rst

@@ -1342,8 +1342,8 @@ follow::
 
 In addition to read/modify/write the setup header of the struct
 boot_params as that of 16-bit boot protocol, the boot loader should
-also fill the additional fields of the struct boot_params as that
-described in zero-page.txt.
+also fill the additional fields of the struct boot_params as
+described in chapter :doc:`zero-page`.
 
 After setting up the struct boot_params, the boot loader can load the
 32/64-bit kernel in the same way as that of 16-bit boot protocol.
@@ -1379,7 +1379,7 @@ can be calculated as follows::
 In addition to read/modify/write the setup header of the struct
 boot_params as that of 16-bit boot protocol, the boot loader should
 also fill the additional fields of the struct boot_params as described
-in zero-page.txt.
+in chapter :doc:`zero-page`.
 
 After setting up the struct boot_params, the boot loader can load
 64-bit kernel in the same way as that of 16-bit boot protocol, but