Clarify psa_destroy_key documentation

Co-authored-by: Janos Follath <janos.follath@arm.com>
Signed-off-by: Ryan Everett <144035422+Ryan-Everett-arm@users.noreply.github.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 565b5e1..56265c1 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1118,7 +1118,7 @@
 
 #if defined(MBEDTLS_PSA_CRYPTO_STORAGE_C)
     if (!PSA_KEY_LIFETIME_IS_VOLATILE(slot->attr.lifetime)) {
-        /* Destroy the copy of the persistent key from memory.
+        /* Destroy the copy of the persistent key from storage.
          * The slot will still hold a copy of the key until the last reader
          * unregisters. */
         status = psa_destroy_persistent_key(slot->attr.id);