fix

Author: yutakahirano

fetch.bs

@@ -4252,7 +4252,7 @@ method, when invoked, must run these steps:
 <h3 id=body-mixin>Body mixin</h3>
 
 <pre class=idl>
-typedef (Blob or BufferSource or FormData or URLSearchParams or USVString) BodyInit;</pre>
+typedef (Blob or BufferSource or FormData or URLSearchParams or USVString or ReadableStream) BodyInit;</pre>
 
 <p>To <dfn id=concept-bodyinit-extract for=BodyInit export>extract</dfn> a <a for=/>body</a> and a
 `<code>Content-Type</code>` <a for=header>value</a> from