Fix variable in Request constructor

_input_ is either a string or a request. Since this links to body (through unusable), it is supposed to be _inputBody_.

Author: TimvdLippe

fetch.bs

@@ -8868,7 +8868,7 @@ constructor steps are:
   <p>If <var>initBody</var> is null and <var>inputBody</var> is non-null, then:
 
   <ol>
-   <li><p>If <var>input</var> is <a for=Body>unusable</a>, then <a>throw</a> a {{TypeError}}.
+   <li><p>If <var>inputBody</var> is <a for=Body>unusable</a>, then <a>throw</a> a {{TypeError}}.
 
    <!-- Any steps after this must not throw. -->