fix(serializers.splunkmetric): Fix TOML option name for multi-metric (#13511)

plugins/serializers/splunkmetric/README.md

@@ -91,6 +91,7 @@ to manage the HEC authorization, here's a sample config for an HTTP output:
   ## more about them here:
   ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_OUTPUT.md
   data_format = "splunkmetric"
+
   ## Provides time, index, source overrides for the HEC
   splunkmetric_hec_routing = true
   # splunkmetric_multimetric = true

plugins/serializers/splunkmetric/splunkmetric.go

@@ -10,7 +10,7 @@ import (
 
 type Serializer struct {
 	HecRouting   bool `toml:"splunkmetric_hec_routing"`
-	MultiMetric  bool `toml:"splunkmetric_multi_metric"`
+	MultiMetric  bool `toml:"splunkmetric_multimetric"`
 	OmitEventTag bool `toml:"splunkmetric_omit_event_tag"`
 }