Update CHANGELOG (#1197)

Author: Joao-Dionisio

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Added `getMemUsed()`, `getMemTotal()`, and `getMemExternEstim()` methods
 ### Fixed
 - Removed `Py_INCREF`/`Py_DECREF` on `Model` in `catchEvent`/`dropEvent` that caused memory leak for imbalanced usage
+- Used getIndex() instead of ptr() for sorting nonlinear expression terms to avoid nondeterministic behavior 
 ### Changed
 ### Removed
 - Removed outdated warning about Make build system incompatibility