doxygen: \p commands misuse - review comments

Apply comments suggested in review.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index 2a2b64a..56fc2d8 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -779,7 +779,7 @@
  * \param ctx           The generic cipher context. This must be initialized and
  *                      bound to a key.
  * \param output        The buffer to write data to. This needs to be a writable
- *                      buffer of at least \c block_size Bytes.
+ *                      buffer of at least block_size Bytes.
  * \param olen          The length of the data written to the \p output buffer.
  *                      This may not be \c NULL.
  *