pki: Fixed documentation about return value.

Documentation now is congruent with the code:
- SSH_OK is returned on success,
- SSH_ERROR is returned on error.

Signed-off-by: Younes Serraj <younes.serraj@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 66c6ae1a55)

src/pki.c

@@ -1094,7 +1094,7 @@ error:
  * @param[out] pkey     A pointer to store the newly allocated public key. You
  *                      NEED to free the key.
  *
- * @return              A public key, NULL on error.
+ * @return              SSH_OK on success, SSH_ERROR on error.
  *
  * @see ssh_key_free()
  */