When trying to use a class pattern to match on a non-class object, we
currently get the following error message:

    Traceback (most recent call last):
      File "/tmp/test.py", line 2, in <module>
        case len():
             ~~~^^
    TypeError: called match pattern must be a class

The terminology "called match pattern" here is a bit confusing. It's not
used anywhere else as far as I can tell, and for a beginner, it might
read like something is actually being called here.

The documentation consistently refers to these patterns as "class
patterns":

* https://docs.python.org/3/reference/compound_stmts.html#class-patterns
* https://peps.python.org/pep-0635/#class-patterns
* https://docs.python.org/3/library/ast.html#ast.MatchClass
* https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching

Here, we change this message to "class pattern must refer to a class".