cue/ast: fix typo

Change-Id: Ief441668de98358a724adf751652c0901679f236
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6480
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/ast/ast.go

@@ -725,7 +725,7 @@ type BinaryExpr struct {
 }
 
 // NewBinExpr creates for list of expressions of length 2 or greater a chained
-// binary expression of the form (((x1 op x2) op x3) ...). For lists of lenght
+// binary expression of the form (((x1 op x2) op x3) ...). For lists of length
 // 1 it returns the expression itself. It panics for empty lists.
 // Useful for ASTs generated by code other than the CUE parser.
 func NewBinExpr(op token.Token, operands ...Expr) Expr {