Fix hermes-engine in Nightlies

Summary:
With commit [332be0f](332be0f) nightlies were broken due to a wrong update with the method we need to use to download hermes.

This change fixes that issue

## Changelog:
[Internal] - Fix hermes-engine download in Nightlies

Reviewed By: cortinico

Differential Revision: D46800717

fbshipit-source-id: 62ce94b980110f46337ce2520fa3c9fbcfe856cc

Author: cipolleschi

packages/react-native/sdks/hermes-engine/hermes-utils.rb

@@ -131,7 +131,7 @@ def download_hermes_tarball(react_native_path, tarball_url, version, configurati
 # Returns: the path to the downloaded Hermes tarball
 def download_nightly_hermes(react_native_path, version)
     tarball_url = nightly_tarball_url(version)
-    return download_stable_hermes(react_native_path, tarball_url, version, nil)
+    return download_hermes_tarball(react_native_path, tarball_url, version, nil)
 end
 
 def nightly_tarball_url(version)