Doc: admin-guide: use correct legends in kernel-parameters.txt

Doc: admin-guide: use correct legends in kernel-parameters.txt

Documentation/admin-guide/kernel-parameters.rst includes a legend
telling us what configurations or hardware platforms are relevant
for certain boot options.  For X86, it is spelled "X86" and for
x86_64, it is spelled "X86-64", so make corrections for those.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Link: https://lore.kernel.org/r/20200810024941.30231-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>