cue/ast: fix typo in docstring
Signed-off-by: Brandon Bloom <brandon.d.bloom@gmail.com>
Change-Id: I5d17d779ff3613a7f30a636ccb826161f1a5cc29
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/532192
Reviewed-by: Paul Jolly <paul@myitcv.io>
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