minor #5633 Upgrade Platform.sh configuration snippet. (GuGuss)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5633).

Discussion
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Upgrade Platform.sh configuration snippet.

Following a recent Platform.sh upgrade: https://docs.platform.sh/user_guide/reference/upgrade/index.html

Commits
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df0afdd Upgrade Platform.sh configuration snippet.

Author: xabbuh

cookbook/deployment/platformsh.rst

@@ -37,9 +37,11 @@ Platform.sh how to deploy your application (read more about
 
     # The name of this app. Must be unique within a project.
     name: myphpproject
-
-    # The toolstack used to build the application.
-    toolstack: "php:symfony"
+    
+    # The type of the application to build.
+    type: php:5.6
+    build:
+      flavor: symfony
 
     # The relationships of the application with services or other applications.
     # The left-hand side is the name of the relationship as it will be exposed