Apply imperative style in the changelog entry
diff --git a/ChangeLog b/ChangeLog
index 26c7cd3..f067bd3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,8 +22,8 @@
Inserted as an enhancement for #1371
* Add support for alternative CSR headers, as used by Microsoft and defined
in RFC 7468. Found by Michael Ernst. Fixes #767.
- * 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
