point to mint v1.4.0 for HTTP/2 extended connect support

Author: the-mikedavis

README.md

@@ -58,18 +58,15 @@ If `Mint.WebSocket.upgrade/4` returns
 Then the server does not support HTTP/2 WebSockets or does not have them
 enabled.
 
-HTTP/2 support for extended CONNECT has been merged into Mint's main branch
-but is not yet published. If you need HTTP/2 support, use an override:
+Support for HTTP/2 extended CONNECT was added to Mint in version `1.4.0`.
+If you need HTTP/2 support, make sure you require that version as a minimum.
 
 ```elixir
 # mix.exs
 def deps do
   [
     {:mint_web_socket, "~> 0.1"},
-    {:mint,
-     git: "https://github.com/elixir-mint/mint.git",
-     ref: "488a6ba5fd418a52f697a8d5f377c629ea96af92",
-     override: true},
+    {:mint, "~> 1.4"},
     # ..
   ]
 end