Fix troubleshooting guide debugging section

Kibana index name prefix to create was wrong.
As describe in
'kolla-ansible/ansible/roles/common/templates/conf/output/01-es.conf.j2'
index prefix is "flog".

Change-Id: I6c2a38808258a8968024bcf8992353ff9672e0f9

doc/troubleshooting.rst

@@ -95,7 +95,7 @@ values are overridden, in ``/etc/kolla/globals.yml``. The value of
 ``<kibana_password>`` can be found in ``/etc/kolla/passwords.yml``.
 
 .. note:: When you log in to Kibana web interface for the first time, you are
-   prompted to create an index. Please create an index using the name ``log-*``.
+   prompted to create an index. Please create an index using the name ``flog-*``.
    This step is necessary until the default Kibana dashboard is implemented in
    Kolla.