guides: apt-snapshot-policy: Add snapshot policy example

The example is missing, add it.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

content/guides/apt-snapshot-policy.md

@@ -91,7 +91,19 @@ will be dropped on each run.
 
 #### Example policy configuration
 
-For example, the above policy:
+```
+$ cat /srv/obs/aptly/snapshot.policy.d/apertis.v202x.yaml
+prefix: shared/apertis/public/apertis
+distribution: v202.*
+retention:
+  days_range: 7
+  weeklies_range: 4
+  monthlies_range: 6
+  keep:
+    - .*/202104.*
+    - v2022/20220613T012229Z
+    - v2023/20230614T142755Z
+```
 
 * Apply this policy to all snapshots from prefix `shared/apertis/public/apertis`
   and distribution matching regex `v202.*`.