Update control startup example

This commit updates control startup example to prevent Compose from interpolating values intended as shell command arguments.

Author: vpavic

content/manuals/compose/how-tos/startup-order.md

@@ -42,7 +42,7 @@ services:
   db:
     image: postgres
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
+      test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
       interval: 10s
       retries: 5
       start_period: 30s