docs: fix Installation documentation (aquasecurity#1804)

* Add missing `image` argument in the Installation docs->Docker section for MacOS.

docs/getting-started/installation.md

@@ -124,7 +124,7 @@ Example:
 === "macOS"
 
     ``` bash
-    docker run --rm -v $HOME/Library/Caches:/root/.cache/ aquasec/trivy:{{ git.tag[1:] }} python:3.4-alpine
+    docker run --rm -v $HOME/Library/Caches:/root/.cache/ aquasec/trivy:{{ git.tag[1:] }} image [YOUR_IMAGE_NAME
     ```
 
 If you would like to scan the image on your host machine, you need to mount `docker.sock`.