fix link of license badge

master branch has been renamed to main.

README.md

@@ -7,7 +7,7 @@
 [![Docker Pulls](https://img.shields.io/docker/pulls/vaultwarden/server.svg)](https://hub.docker.com/r/vaultwarden/server)
 [![Dependency Status](https://deps.rs/repo/github/dani-garcia/vaultwarden/status.svg)](https://deps.rs/repo/github/dani-garcia/vaultwarden)
 [![GitHub Release](https://img.shields.io/github/release/dani-garcia/vaultwarden.svg)](https://github.com/dani-garcia/vaultwarden/releases/latest)
-[![GPL-3.0 Licensed](https://img.shields.io/github/license/dani-garcia/vaultwarden.svg)](https://github.com/dani-garcia/vaultwarden/blob/master/LICENSE.txt)
+[![GPL-3.0 Licensed](https://img.shields.io/github/license/dani-garcia/vaultwarden.svg)](https://github.com/dani-garcia/vaultwarden/blob/main/LICENSE.txt)
 [![Matrix Chat](https://img.shields.io/matrix/vaultwarden:matrix.org.svg?logo=matrix)](https://matrix.to/#/#vaultwarden:matrix.org)
 
 Image is based on [Rust implementation of Bitwarden API](https://github.com/dani-garcia/vaultwarden).