Point docker hub badge to correct repository

The link is already pointing to the new image location, but the badge
source was somewhat confusingly showing stats for the old image.

README.md

@@ -4,7 +4,7 @@
 
 ---
 
-[![Docker Pulls](https://img.shields.io/docker/pulls/bitwardenrs/server.svg)](https://hub.docker.com/r/vaultwarden/server)
+[![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)