Add colon to docker-compose volume name

The volume name for the postgres data volume was missing a semicolon. This update should make copy/paste of the `docker-compose.yml` for postgres a little easier.

Author: ericpoe

getting-started/installation/installation.md

@@ -87,7 +87,7 @@ services:
         volumes:
             - speedtest-db:/var/lib/postgresql/data
 volumes:
-  speedtest-app
+  speedtest-app:
   speedtest-db:
 ```
 {% endtab %}