bench: Fix path to default sample file

The sample file used for benchmarking was renamed in 8326e41 but
the `--file` flag default was not updated.

cmd/tsdb/main.go

@@ -43,7 +43,7 @@ func main() {
 		benchWriteCmd        = benchCmd.Command("write", "run a write performance benchmark")
 		benchWriteOutPath    = benchWriteCmd.Flag("out", "set the output path").Default("benchout/").String()
 		benchWriteNumMetrics = benchWriteCmd.Flag("metrics", "number of metrics to read").Default("10000").Int()
-		benchSamplesFile     = benchWriteCmd.Arg("file", "input file with samples data, default is (../../testdata/20k.series)").Default("../../testdata/20k.series").String()
+		benchSamplesFile     = benchWriteCmd.Arg("file", "input file with samples data, default is (../../testdata/20kseries.json)").Default("../../testdata/20kseries.json").String()
 		listCmd              = cli.Command("ls", "list db blocks")
 		listPath             = listCmd.Arg("db path", "database path (default is benchout/storage)").Default("benchout/storage").String()
 	)