Fix typo: inferface. (#821)

Author: Ms2ger

index.bs

@@ -12618,7 +12618,7 @@ The \[[Prototype]] [=internal slot=] of an [=asynchronous iterator prototype obj
 
 <div algorithm="to invoke the return property of asynchronous iterators">
 
-    If an [=asynchronous iterator return=] algorithm is defined for the [=inferface=], then the
+    If an [=asynchronous iterator return=] algorithm is defined for the [=interface=], then the
     [=asynchronous iterator prototype object=] must have a <code class="idl">return</code> data
     property with attributes { \[[Writable]]: <emu-val>true</emu-val>,
     \[[Enumerable]]: <emu-val>true</emu-val>, \[[Configurable]]: <emu-val>true</emu-val> }