src/sage/combinat/symmetric_group_algebra.py: add long time

Fix the CI warning,

  2025-07-14T18:32:50.5528101Z ##[warning]slow doctest:
  2025-07-14T18:32:50.5529322Z     for la in Partitions(SGA.n):
  2025-07-14T18:32:50.5529772Z         idem = SGA.ladder_idempotent(la)
  2025-07-14T18:32:50.5530453Z         assert idem^2 == idem
  2025-07-14T18:32:50.5530907Z         print(la, SGA.principal_ideal(idem).dimension())
  2025-07-14T18:32:50.5531396Z Test ran for 5.81s cpu, 5.89s wall

by adding "# long time" to this test.

Author: orlitzky

src/sage/combinat/symmetric_group_algebra.py

@@ -1313,6 +1313,7 @@ def ladder_idempotent(self, la):
         modules (which are the :meth:`Specht modules <specht_module>`
         and also projective modules)::
 
+            sage: # long time
             sage: SGA = SymmetricGroupAlgebra(QQ, 5)
             sage: for la in Partitions(SGA.n):
             ....:     idem = SGA.ladder_idempotent(la)