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  .