Update spec.bs

Co-authored-by: qingxinwu <[email protected]>

spec.bs

@@ -2669,7 +2669,7 @@ is a [=structured header=] whose value must be an [=structured header/integer=].
 <div algorithm>
 To <dfn>fetch trusted signals</dfn> given a [=URL=] |url|, an [=origin=] |scriptOrigin|, a
 [=policy container=] |policyContainer|, and a [=boolean=] |isBiddingSignal|. They return a
-[=tuple=] consisting of ([=ordered map=] or null, [=ordered map=] or null, integer):
+[=tuple=] consisting of ([=ordered map=] or null, [=ordered map=] or null, integer or null):
 
   1. Let |request| be a new [=request=] with the following properties:
     :   [=request/URL=]