Update README.md

Error shard name (toml) doesn't match dependency name (crystal-toml)

README.md

@@ -10,7 +10,7 @@ Add this to your application's `shard.yml`:
 
 ```yaml
 dependencies:
-  crystal-toml:
+  toml:
     github: manastech/crystal-toml
     branch: master
 ```