fix typo

Author: lesnik512

README.md

@@ -248,7 +248,7 @@ class YourFastApiSettings(FastApiSettings):
 
 Parameters description:
 
-- `service_name` - will be attached to metric's names, but has to be named in [snake_case](https://en.wikipedia.org/wiki/Snake_case).
+- `service_name` - will be attached to metrics's names, but has to be named in [snake_case](https://en.wikipedia.org/wiki/Snake_case).
 - `prometheus_metrics_path` - path to metrics handler.
 - `prometheus_metrics_include_in_schema` - whether to include metrics route in OpenAPI schema.
 - `prometheus_instrumentator_params` - will be passed to `Instrumentor` during initialization.