[install-guide] change "API endpoints" to "API endpoint"

An API endpoint on Keystone uses several URLs. We normally (not for
heat) only have one API endpoint for a service in the installation guide.

Change-Id: Idb22a6e0c06382e9e8069eed6e16e69c9c512bb3

doc/install-guide/section_ceilometer-controller.xml

@@ -17,7 +17,7 @@
     <title>To configure prerequisites</title>
     <para>Before you install and configure Telemetry, you must install
       <application>MongoDB</application>, create a MongoDB database,
-      service credentials, and API endpoints.</para>
+      service credentials, and API endpoint.</para>
     <step os="opensuse;sles">
       <para>Enable the Open Build Service repositories for MongoDB based on
         your openSUSE or SLES version:</para>
@@ -134,7 +134,7 @@ connecting to: controller:27017/test
       </substeps>
     </step>
     <step>
-      <para>Create the Telemetry module API endpoints:</para>
+      <para>Create the Telemetry module API endpoint:</para>
       <screen><prompt>$</prompt> <userinput>keystone endpoint-create \
   --service-id $(keystone service-list | awk '/ metering / {print $2}') \
   --publicurl http://<replaceable>controller</replaceable>:8777 \