Fix typespec (#1)

This tiny correction helps apps that use this library directly to pass Dialyzer.

Author: x0id

src/prometheus_http_config.erl

@@ -42,7 +42,7 @@
 path() ->
     get_value(path, ?DEFAULT_PATH).
 
--spec valid_path_and_registry(string(), prometheus_registry:registry()) ->
+-spec valid_path_and_registry(true | string(), prometheus_registry:registry()) ->
     false
     | {true, prometheus_registry:registry()}
     | {registry_conflict, prometheus_registry:registry(), prometheus_registry:registry()}