Correct request's window check

It's either "no-window" or an environment settings object.

Author: annevk

fetch.bs

@@ -4312,7 +4312,7 @@ the response. [[!HTTP-CACHING]]
    <li><p><var>request</var>'s <a for=request>mode</a> is "<code>same-origin</code>",
    "<code>cors</code>", or "<code>no-cors</code>"
 
-   <li><p><var>request</var>'s <a for=request>window</a> is non-null
+   <li><p><var>request</var>'s <a for=request>window</a> is an <a>environment settings object</a>
 
    <li><p><var>request</var>'s <a for=request>method</a> is `<code>GET</code>`