fix doctest in doc

Author: fchapoton

src/doc/en/thematic_tutorials/coercion_and_categories.rst

@@ -130,7 +130,6 @@ This base class provides a lot more methods than a general parent::
      'gen',
      'gens',
      'integral_closure',
-     'is_commutative',
      'is_field',
      'krull_dimension',
      'ngens',