Update get-usage-metrics.mdx (#566)

cURL authorization needed "Bearer" in the header.

Author: ecmulli

api-reference/viewership/get-usage-metrics.mdx

@@ -8,7 +8,7 @@ openapi: "GET /data/usage/query"
 ```bash cURL
 curl --request GET \
   --url https://livepeer.studio/api/data/usage/query \
-  --header 'Authorization: <api-key>'
+  --header 'Authorization: Bearer <api-key>'
 ```
 
 ```javascript JavaScript