Update HighModelInvocationErrors.yml

I was using count in a summary metric. Modified to sum to show the actual value of the metric. 
Before it was showing the number of times the event was aggregated. Ref here: https://docs.newrelic.com/docs/data-apis/understand-data/metric-data/metric-data-type/

alert-policies/amazon-sagemaker/HighModelInvocationErrors.yml

@@ -5,7 +5,7 @@ description: |+
 
 type: STATIC
 nrql:
-  query: "SELECT count(`aws.sagemaker.InvocationModelErrors`) as 'Query' FROM Metric"
+  query: "SELECT sum(`aws.sagemaker.InvocationModelErrors`) as 'Query' FROM Metric"
 
 # Function used to aggregate the NRQL query value(s) for comparison to the terms.threshold (Default: SINGLE_VALUE)
 valueFunction: SINGLE_VALUE
@@ -24,4 +24,4 @@ terms:
 
 # Duration after which a violation automatically closes
 # Time in seconds; 300 - 2592000 (Default: 86400 [1 day])
-violationTimeLimitSeconds: 86400
+violationTimeLimitSeconds: 86400