Adjust documentation of mbedtls_cipher_update_ad
diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index 4a71383..2d609db 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -682,7 +682,7 @@
* \param ctx The generic cipher context. This must be initialized.
* \param ad The additional data to use. This must be a readable
* buffer of at least \p ad_len Bytes.
- * \param ad_len The Length of \p ad Bytes.
+ * \param ad_len The length of \p ad in Bytes.
*
* \return \c 0 on success.
* \return A specific error code on failure.