Improve wording in the ChangeLog
diff --git a/ChangeLog b/ChangeLog
index 7bd547a..2f69ecf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,8 +3,8 @@
 = mbed TLS 2.7.x branch released xxxx-xx-xx
 
 Changes
-   * Reduced the complexity of the timing tests, as they were failing in less
-     reliable environments.
+   * Reduced the complexity of the timing tests, as they were assuming more than
+     the underlying OS actually guarantees.
 
 = mbed TLS 2.7.9 branch released 2018-12-21