Delete duplicated record (fluent#611)

the doc has a duplicated options tag_prefix

Signed-off-by: Evgeniy Shubin <evgeniy.shubin.android@gmail.com>

Author: com30n

pipeline/outputs/stackdriver.md

@@ -28,7 +28,6 @@ Before to get started with the plugin configuration, make sure to obtain the pro
 | labels\_key | The value of this field is used by the Stackdriver output plugin to find the related labels from jsonPayload and then extract the value of it to set the LogEntry Labels. | logging.googleapis.com/labels |
 | tag\_prefix | Set the tag\_prefix used to validate the tag of logs with k8s resource type. Without this option, the tag of the log must be in format of k8s\_container\(pod/node\).\* in order to use the k8s\_container resource type. Now the tag prefix is configurable by this option \(note the ending dot\). | k8s\_container., k8s\_pod., k8s\_node. |
 | severity\_key | Specify the name of the key from the original record that contains the severity information. |  |
-| tag\_prefix | Set the tag\_prefix used to validate the tag of logs with k8s resource type. Without this option, the tag of the log must be in format of k8s\_container\(pod/node\).\* in order to use the k8s\_container resource type. Now the tag prefix is configurable by this option. | k8s\_container., k8s\_pod., k8s\_node. |
 
 ### Configuration File