Fix typo for property name (#15380)

Co-authored-by: Arvind Krishnaa Jagannathan <arjagann@microsoft.com>

Author: arv100kri

specification/search/resource-manager/Microsoft.Search/preview/2021-04-01-preview/search.json

@@ -2389,7 +2389,7 @@
     "OperationServiceSpecification": {
       "description": "Specifications of the service for this operation.",
       "properties": {
-        "metricsSpecifications": {
+        "metricSpecifications": {
           "type": "array",
           "readOnly": true,
           "items": {