Fix incorrect method name for save_as_heic in README

[laymonage]

The actual method is save_as_heic:

Willow/willow/plugins/pillow.py

Lines 378 to 387 in 84ab87f

	@Image.operation
	def save_as_heic(
	self,
	f,
	quality: int = 80,
	lossless: bool = False,
	apply_optimizers: bool = True,
	):
	"""
	Save the image as a HEIC file.

https://willow.wagtail.org/latest/guide/save.html#lossless-avif-heic-and-webp