Update future Mbed TLS version in Changelog

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog b/ChangeLog
index 6e3467f..3571910 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS x.x.x branch released xxxx-xx-xx
+= Mbed TLS 3.0.0 branch released 2021-xx-xx
 
 API changes
    * Remove HAVEGE module.