replace master with main

Author: lionello

README.md

@@ -11,7 +11,7 @@ This repo includes:
 * Read our [Terms and Conditions](https://defang.io/terms-service.html)
 * Download the [latest version](https://github.com/defang-io/defang/releases/latest/) of the Defang CLI. For this beta, MacOS users will have to explicitly allow running of downloaded programs in the OS security settings.
   * or use the [Nix package manager](https://nixos.org):
-    * with Nix-Env: `nix-env -if https://github.com/defang-io/defang/archive/master.tar.gz`
+    * with Nix-Env: `nix-env -if https://github.com/defang-io/defang/archive/main.tar.gz`
     * with Flakes: `nix profile install github:defang-io/defang#defang-bin --refresh`
 * Take a look at our [Samples folder](https://github.com/defang-io/defang/tree/main/samples) for example projects in various programming languages.
 * Try the AI integration by running `defang generate`