backups.md: Add missing step in the procedure to enable GFS backups

Signed-off-by: Thomas Moraine <[email protected]>

docs/docs/backups.md

@@ -404,6 +404,7 @@ The section called **Long-term retention of backups** appears.
 - **Daily backups** (Son): The number of daily backups to keep.
 - **Weekly backups** (Father): The number of weekly backups to keep.
 - **Monthly backups** (Grandfather): The number of monthly backups to keep.
+5. Click the **Save** button.
 
 During each backup run, Xen Orchestra evaluates existing backups and removes any excess backups based on the configured policy.