[ci] replace diff by update in wiki

.gitlab-ci.yml

@@ -53,7 +53,7 @@ wikigen:
         DEBIAN_VERSION: [bookworm]
   script:
     - echo "$GRID5000_API" > "${HOME}/.grid5000_api.yml"
-    - bundle exec rake gen:wiki NAME=${GENERATOR} SITE=${SITE} DO=diff
+    - bundle exec rake gen:wiki NAME=${GENERATOR} SITE=${SITE} DO=update
   only:
     refs:
       - master