docs: typos in code block

[juleswg23]

There is an unintended 4th quote closing the docstring, and no colon on the following line.

great-tables/docs/get-started/extensions.qmd

Lines 71 to 74 in a59301b

	def fmt_threshold(gt, columns, threshold: float):
	"""Format values to 0 if they are under threshold.""""
	def _truncate(val: float)
	return "0" if val < threshold else str(val)