Fix spelling of 'reasonable' in comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/ssl_ticket.h b/include/mbedtls/ssl_ticket.h
index a882eed..8221051 100644
--- a/include/mbedtls/ssl_ticket.h
+++ b/include/mbedtls/ssl_ticket.h
@@ -101,7 +101,7 @@
  *                  supported. Usually that means a 256-bit key.
  *
  * \note            The lifetime of the keys is twice the lifetime of tickets.
- *                  It is recommended to pick a reasonnable lifetime so as not
+ *                  It is recommended to pick a reasonable lifetime so as not
  *                  to negate the benefits of forward secrecy.
  *
  * \return          0 if successful,