Misleading "not" typo in assertion in pandas.concat in join_axes check

[Ignalion]

There is an assertion message in concat.py:

            if len(self.join_axes) != ndim - 1:
                raise AssertionError("length of join_axes must not be equal "
                                     "to {length}".format(length=ndim - 1))

From code (and from it's usage) it's obvious that word "not" is incorrect here and should be deleted.

[TomAugspurger]

Thanks for the report. Interested in submitting a PR fixing it?

[rdmontgomery]

Hi there. I submitted a PR to remove the "not" from the assertion message. This is my first contribution to pandas! Happy to follow up if I missed anything in the process.