fix getting navigable

Author: OrKoN

index.bs

@@ -2910,10 +2910,13 @@ The [=remote end steps=] with <var ignore>session</var> and |command parameters|
 1. Let |navigable id| be the value of the <code>context</code> field of
    |command parameters| if present, or null otherwise.
 
-1. If the implementation is unable to capture a screenshot of |navigable id| for any
+1. Let |navigable| be the result of [=trying=] to [=get a navigable=]
+    with |navigable id|.
+
+1. If the implementation is unable to capture a screenshot of |navigable| for any
    reason then return [=error=] with [=error code=] [=unsupported operation=].
 
-1. Let |document| be |navigable id|'s [=active document=].
+1. Let |document| be |navigable|'s [=active document=].
 
 1. Immediately after the next invocation of the [=run the animation frame
    callbacks=] algorithm for |document|: