Documentation/x86: Fix backwards on/off logic about YMM support

These options clearly turn *off* XSAVE YMM support.  Correct the
typo.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 553a5c03 ("x86/speculation: Add force option to GDS mitigation")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>