KVM: Documentation: Fix documentation for nested.

Nested VMX was enabled by default in commit 1e58e5e5

 ("KVM:
VMX: enable nested virtualization by default"), which was merged
in Linux 4.20. This patch is to fix the documentation accordingly.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Message-Id: <20210128154747.4242-1-yu.c.zhang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>