fix doctest in doc

Author: fchapoton

src/doc/en/thematic_tutorials/coercion_and_categories.rst

@@ -132,7 +132,6 @@ This base class provides a lot more methods than a general parent::
      'ngens',
      'one',
      'order',
-     'random_element',
      'zero',
      'zeta',
      'zeta_order']