fix: fix broken links (#7)

README.md

diff --git a/README.md b/README.md
index 1a9481a..c20ba4a 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # Rocket Casbin Middleware
 
-[![Crates.io](https://meritbadge.herokuapp.com/rocket-authz)](https://crates.io/crates/rocket-authz)
+[![Crates.io](https://img.shields.io/crates/d/rocket-authz)](https://crates.io/crates/rocket-authz)
 [![Docs](https://docs.rs/rocket-authz/badge.svg)](https://docs.rs/rocket-authz)
 [![CI](https://github.com/casbin-rs/rocket-authz/workflows/CI/badge.svg)](https://github.com/casbin-rs/rocket-authz/actions)
 [![codecov](https://codecov.io/gh/casbin-rs/rocket-authz/branch/master/graph/badge.svg)](https://codecov.io/gh/casbin-rs/rocket-authz)