[with patch, positive review] matrix0.pyx: fix doctest for commutator

[jhpalmieri]

Here is the extent of the docstring for the commutator method in matrix0.pyx:

        Return the commutator self*other - other*self.

        EXAMPLES:
            sage: A = Matrix(QQ[['t']], 2, 2, range(4))

Fix the doctest so that it actually computes a commutator.

[cwitty]

New doctests look good, and they pass.

(Good catch on noticing the original bug, too.)

Positive review.

[mabshoff]

Merged in Sage 3.3.alpha6.

Cheers,

Michael