Apply imperiative style in the changelog entry
diff --git a/ChangeLog b/ChangeLog
index c419274..d16675d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,8 +13,8 @@
in the header files, which missed the precompilation check. #971
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 2.16.0 branch released 2018-12-21