Update index.bs

Co-authored-by: Julian Descottes <[email protected]>

Author: sadym-chromium

index.bs

@@ -6365,7 +6365,7 @@ The <dfn export>WebDriver BiDi emulated User-Agent</dfn> steps given
 The [=remote end steps=] given |session| and |command parameters| are:
 
 1. If |command parameters| [=map/contains=] "<code>userContexts</code>"
-   and |command parameters| [=map/contains=] "<code>context</code>",
+   and |command parameters| [=map/contains=] "<code>contexts</code>",
    return [=error=] with [=error code=] [=invalid argument=].
 
 1. Let |emulated user agent| be null.