fix typo pointed out by @slivingston · python-control/python-control@dc74493

Original file line number	Diff line number	Diff line change
`@@ -470,7 +470,7 @@ def __str__(self, var=None):`
`470`	`470`	`elif self.display_format == 'zpk':`
`471`	`471`	`num = self.num[no][ni]`
`472`	`472`	`if num.size == 1 and num.item() == 0:`
`473`		`-# Catch a special case taht SciPy doesn't handle`
	`473`	`+# Catch a special case that SciPy doesn't handle`
`474`	`474`	`z, p, k = tf2zpk([1.], self.den[no][ni])`
`475`	`475`	`k = 0`
`476`	`476`	`else:`