Fix incorrect installation docs

`unleash_ex` did return an error form hex:
Failed to fetch record for 'hexpm/unleash_ex' from registry (using cache instead)
This could be because the package does not exist, it was spelled incorrectly or you don't have permissions to it
** (Mix) No package with name unleash_ex (from: mix.exs) in registry

README.md

@@ -28,7 +28,7 @@ by adding `unleash_ex` to your list of dependencies in `mix.exs`:
 ```elixir
 def deps do
   [
-    {:unleash_ex, "~> 1.0"}
+    {:unleash, "~> 1.0"}
   ]
 end
 ```