Reword IAR changelog for fixing compiler warnings

Signed-off-by: Agathiyan Bragadeesh <agathiyan.bragadeesh2@arm.com>
diff --git a/ChangeLog.d/fix-iar-compiler-warnings.txt b/ChangeLog.d/fix-iar-compiler-warnings.txt
index eefbd12..50f1dca 100644
--- a/ChangeLog.d/fix-iar-compiler-warnings.txt
+++ b/ChangeLog.d/fix-iar-compiler-warnings.txt
@@ -1,2 +1,2 @@
 Bugfix
-   * Improve general IAR support
+   * Fix IAR compiler warnings. Fixes #7873, #4300.