Fix parameter name in doxygen
@@ -3904,8 +3904,8 @@ void mbedtls_ssl_free( mbedtls_ssl_context *ssl );
* \param ssl The SSL context to save. On success, it is no longer
* associated with the connection that has been serialized.
* \param buf The buffer to write the serialized data to. It must be a
- * writeable buffer of at least \p len bytes, or may be \c
- * NULL if \p len is \c 0.
+ * writeable buffer of at least \p buf_len bytes, or may be \c
+ * NULL if \p buf_len is \c 0.
* \param buf_len The number of bytes available for writing in \p buf.
* \param olen The size in bytes of the data that has been or would have
* been written. It must point to a valid \c size_t.
