[gimp-web] devel-docs: update the mirror howto with a verification step.

[Jehan <jehanp src gnome org>]

commit 7393490ac0b0ce71899eddaa4216dc78c821582b
Author: Jehan <jehan girinstud io>
Date:   Tue Oct 5 12:25:01 2021 +0200

    devel-docs: update the mirror howto with a verification step.

 devel-docs/mirror-howto.md | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/devel-docs/mirror-howto.md b/devel-docs/mirror-howto.md
index 37f847ac..c2cff7fc 100644
--- a/devel-docs/mirror-howto.md
+++ b/devel-docs/mirror-howto.md
@@ -85,9 +85,14 @@ instance @Jehan can take care of part of the actions:
    other data. In "more", also add the report link.
    If other data needs to be updated, do so as well (for instance if
    other mirrors changed).
-11. Commit all the changes and push them to `gimp-web`'s `testing`
+11. Verify all mirrors (especially new ones) are well synced at least
+    for the last release:
+    ```sh
+    tools/downloads/gimp-check-mirrors.py
+    ```
+12. Commit all the changes and push them to `gimp-web`'s `testing`
     branch.
-12. After a short time, make sure that testing's [sponsor 
page](https://testing.gimp.org/donating/sponsors.html)
+13. After a short time, make sure that testing's [sponsor 
page](https://testing.gimp.org/donating/sponsors.html)
     is properly updated.
 
 Of course, the public website will be updated when you merge `testing`