Fix typo in psa config comment

Change "are" to "aren't" to avoid making the comment misleading.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/include/mbedtls/config_psa.h b/include/mbedtls/config_psa.h
index f744e12..96f6f4a 100644
--- a/include/mbedtls/config_psa.h
+++ b/include/mbedtls/config_psa.h
@@ -253,7 +253,7 @@
 #endif /* PSA_WANT_ALG_STREAM_CIPHER */
 
 /* The MBEDTLS_PSA_HAVE_SOFT_KEY_TYPE_* are defined if a key type is selected,
- * but we are configured to accelerate this key type. */
+ * but we aren't configured to accelerate this key type. */
 #if defined(PSA_WANT_KEY_TYPE_AES) && !defined(MBEDTLS_PSA_ACCEL_KEY_TYPE_AES)
 #define PSA_HAVE_SOFT_KEY_TYPE_AES 1
 #endif