Documentation/x86: Update the naming of CPU features for /proc/cpuinfo

Commit:

  78ce84b9 ("x86/cpufeatures: Flip the /proc/cpuinfo appearance logic")

changed how CPU feature names should be specified. Update document to
reflect the same.

Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250409111341.GDZ_ZWZS4LckBcirLE@fat_crate.local