bpo-42670: Fix a missing word in the itertools.product() docs (GH-23823) (GH-23824)

Author: miss-islington

Doc/library/itertools.rst

@@ -565,7 +565,7 @@ loops that truncate the stream.
 
    Before :func:`product` runs, it completely consumes the input iterables,
    keeping pools of values in memory to generate the products.  Accordingly,
-   it only useful with finite inputs.
+   it is only useful with finite inputs.
 
 .. function:: repeat(object[, times])