doc/ref/spec.md: fix builtin func

Closes #522
https://github.com/cuelang/cue/pull/522

GitOrigin-RevId: 1e9d74b1e625a9426f3d411e91146ce1e1a8ddb7
Change-Id: I2f5f0b492a67845969a4c02e67a3819d9eda913f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7262
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index a022dc1..cbf0f87 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1694,7 +1694,7 @@
 
 ```
 Functions
-len       required  close     open
+len       close and or
 
 Types
 null      The null type and value