tools/kvm_stat: add hint on '-f help' to man page

The man page update for this new functionality was omitted.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/tools/kvm/kvm_stat/kvm_stat.txt b/tools/kvm/kvm_stat/kvm_stat.txt
index 75368a3..b5b3810 100644
--- a/tools/kvm/kvm_stat/kvm_stat.txt
+++ b/tools/kvm/kvm_stat/kvm_stat.txt
@@ -88,7 +88,7 @@
 
 -f<fields>::
 --fields=<fields>::
-	fields to display (regex)
+	fields to display (regex), "-f help" for a list of available events
 
 -h::
 --help::