Editorial: use "tuple" instead of "pair"

Fixes #1057.

Author: saschanaz

index.bs

@@ -4259,7 +4259,7 @@ iteration</dfn> value to signal the end of the iteration, or resolves with one o
 : for [=value asynchronously iterable declarations=]:
 ::  a value of the type given in the declaration;
 : for [=pair asynchronously iterable declarations=]:
-::  a [=pair=] containing a value of the first type given in the declaration, and a value of the second type given in the declaration.
+::  a [=tuple=] containing a value of the first type given in the declaration, and a value of the second type given in the declaration.
 
 The prose may also define an <dfn export>asynchronous iterator return</dfn> algorithm. This
 algorithm receives the instance of the [=interface=] that is being iterated, the async iterator