Update index.d.ts

Author: zigazajc007

index.d.ts

@@ -81,14 +81,14 @@ export class Registry<RegistryContentType = PrometheusContentType> {
 	/**
 	 * Gets the Content-Type of the metrics for use in the response headers.
 	 */
-	readonly contentType: RegistryContentType;
+	readonly contentType: PrometheusContentType | OpenMetricsContentType;
 
 	/**
 	 * Set the content type of a registry. Used to change between Prometheus and
 	 * OpenMetrics versions.
 	 * @param contentType The type of the registry
 	 */
-	setContentType(contentType: RegistryContentType): void;
+	setContentType(contentType: PrometheusContentType | OpenMetricsContentType): void;
 
 	/**
 	 * Merge registers