Fix Typo of "Find Element From Shadow Root" section (#1904)

Author: yezhizhen

index.html

@@ -5302,7 +5302,7 @@ <h4><dfn>Find Element From Shadow Root</dfn></h4>
   <li><p>If <var>selector</var> is <a>undefined</a>,
   return <a>error</a> with <a>error code</a> <a>invalid argument</a>.
 
-  <li><p>If the <var>ssession</var>&apos;s <a>current browsing
+  <li><p>If <var>session</var>&apos;s <a>current browsing
   context</a> is <a>no longer open</a>, return <a>error</a>
   with <a>error code</a> <a>no such window</a>.