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