x86/bugs: Remove apostrophe typo

commit bcf16315 upstream.

Remove a superfluous ' in the mitigation string.

Fixes: e8ec1b6e ("x86/bugs: Enable STIBP for JMP2RET")
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>