docs: switched to serviceName according to #401

Author: dreamorosi

docs/core/metrics.md

@@ -54,7 +54,7 @@ These settings will be used across all metrics emitted:
 Setting | Description | Environment variable | Constructor parameter
 ------------------------------------------------- | ------------------------------------------------- | ------------------------------------------------- | -------------------------------------------------
 **Metric namespace** | Logical container where all metrics will be placed e.g. `serverlessAirline` |  `POWERTOOLS_METRICS_NAMESPACE` | `namespace`
-**Service** | Optionally, sets **service** metric dimension across all metrics e.g. `payment` | `POWERTOOLS_SERVICE_NAME` | `service`
+**Service** | Optionally, sets **service** metric dimension across all metrics e.g. `payment` | `POWERTOOLS_SERVICE_NAME` | `serviceName`
 
 For a **complete list** of supported environment variables, refer to [this section](./../index.md#environment-variables).