Fix grammar: "allows to" → "allows using/blocking" in docs

"Allows to <verb>" is grammatically incorrect in English. The verb
"allow" requires either a gerund ("allows using") or an object +
infinitive ("allows one to use"). Fix two instances in hmac.rst and
asyncio-sync.rst.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Anandesh-Sharma

Doc/library/asyncio-sync.rst

@@ -361,7 +361,7 @@ Barrier
 
    A barrier object.  Not thread-safe.
 
-   A barrier is a simple synchronization primitive that allows to block until
+   A barrier is a simple synchronization primitive that allows blocking until
    *parties* number of tasks are waiting on it.
    Tasks can wait on the :meth:`~Barrier.wait` method and would be blocked until
    the specified number of tasks end up waiting on :meth:`~Barrier.wait`.

Doc/library/hmac.rst

@@ -9,7 +9,7 @@
 --------------
 
 This module implements the HMAC algorithm as described by :rfc:`2104`.
-The interface allows to use any hash function with a *fixed* digest size.
+The interface allows using any hash function with a *fixed* digest size.
 In particular, extendable output functions such as SHAKE-128 or SHAKE-256
 cannot be used with HMAC.