Fix the grammar as suggsted in the review

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23545)

crypto/threads_pthread.c

@@ -399,8 +399,8 @@ void ossl_rcu_read_unlock(CRYPTO_RCU_LOCK *lock)
         }
     }
     /*
-     * if we get here, we're trying to unlock a lock that we never acquired
-     * that's fatal
+     * If we get here, we're trying to unlock a lock that we never acquired -
+     * that's fatal.
      */
     assert(0);
 }
@@ -451,7 +451,7 @@ static struct rcu_qp *update_qp(CRYPTO_RCU_LOCK *lock)
                     __ATOMIC_RELEASE);
 
     /*
-     * update the reader index to be the prior qp
+     * Update the reader index to be the prior qp.
      * Note the use of __ATOMIC_RELEASE here is based on the corresponding use
      * of __ATOMIC_ACQUIRE in get_hold_current_qp, as we want any publication
      * of this value to be seen on the read side immediately after it happens