fix doctest formatting

Author: yyyyx4

src/sage/rings/finite_rings/integer_mod_ring.py

@@ -1366,7 +1366,7 @@ def unit_gens(self, **kwds):
 
         The choice of generators is affected by the optional keyword
         ``algorithm``; this can be ``'sage'`` (default) or ``'pari'``.
-        See :meth:`unit_group` for details.
+        See :meth:`unit_group` for details. ::
 
             sage: A = Zmod(55)
             sage: A.unit_gens(algorithm='sage')