Fix incorrect capitalization in ceilometer install

According to the Conventions document, Ceilometer's official name is
OpenStack Telemetry, so the 'Telemetry Service' should be changed to
'Telemetry service' in install guide.

This patch only fixes the section_ceilometer-install.xml to make it
easier to review.

Change-Id: I1615c16660700dedd0be1eb460b83839142829be
ref: https://wiki.openstack.org/wiki/Documentation/Conventions#Service_and_project_names