Apply imperiative style in the changelog entry
diff --git a/ChangeLog b/ChangeLog
index 2f69ecf..b933ca7 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 assuming more than
-     the underlying OS actually guarantees.
+   * Reduce the complexity of the timing tests. They were assuming more than the
+     underlying OS actually guarantees.
 
 = mbed TLS 2.7.9 branch released 2018-12-21