tools/kvm_stat: Fix documentation typo

Makes the dash in front of option '-z' disappear in the generated
man-page.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Message-Id: <20210506140352.4178789-1-raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>