fix docs

docs/plugins/fluentd/input/sample.md

@@ -8,5 +8,5 @@ The in_sample input plugin generates sample events. It is useful for testing, de
 | tag | The tag of the event. The value is the tag assigned to the generated events. | *string |
 | size | The number of events in the event stream of each emit. | *int64 |
 | rate | It configures how many events to generate per second. | *int64 |
-| auto_increment_key | If specified, each generated event has an auto-incremented key field. | *string |
+| autoIncrementKey | If specified, each generated event has an auto-incremented key field. | *string |
 | sample | The sample data to be generated. It should be either an array of JSON hashes or a single JSON hash. If it is an array of JSON hashes, the hashes in the array are cycled through in order. | *string |