doc/ref/spec.md: fix doc bug in Label production
Change-Id: I3743dd87e788f0eeea060dcabbf3351e75d7bd3e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6145
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index 851714e..9e2efa8 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1121,7 +1121,7 @@
Declaration = Field | Embedding | LetClause | attribute .
Embedding = Comprehension | AliasExpr .
Field = Label ":" { Label ":" } Expression { attribute } .
-Label = identifier "=" LabelExpr .
+Label = [ identifier "=" ] LabelExpr .
LabelExpr = LabelName [ "?" ] | "[" AliasExpr "]" .
LabelName = identifier | simple_string_lit .