Fix documentation

Signed-off-by: David Gageot <david.gageot@docker.com>

Author: dgageot

docs/getting-started/installation/index.md

@@ -34,7 +34,7 @@ Install docker-agent using [Homebrew](https://brew.sh/):
 
 ```bash
 # Install
-$ brew install docker/tap/docker-agent
+$ brew install cagent
 
 # Verify
 $ docker-agent version