Use .tar.gz for test archives

Differential Revision: https://phabricator.services.mozilla.com/D1743

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=733530
gecko-commit: 9c75cab2e32295ee037f0a8f1af2f61ac86f8b8c
gecko-integration-branch: central
gecko-reviewers: gps
diff --git a/README.md b/README.md
index c167935..684f461 100644
--- a/README.md
+++ b/README.md
@@ -133,11 +133,11 @@
 brew install nss
 ```
 
-On other platforms, download the firefox archive and common.tests.zip
+On other platforms, download the firefox archive and common.tests.tar.gz
 archive for your platform from
 [Mozilla CI](https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/).
 
-Then extract `certutil[.exe]` from the tests.zip package and
+Then extract `certutil[.exe]` from the tests.tar.gz package and
 `libnss3[.so|.dll|.dynlib]` and put the former on your path and the latter on
 your library path.