Fix summary.md link to chapter 9

Author: lefticus

SUMMARY.md

@@ -8,7 +8,7 @@
 * [Considering Portability](06-Considering_Portability.md)
 * [Considering Threadability](07-Considering_Threadability.md)
 * [Considering Performance](08-Considering_Performance.md)
-* [Considering Correctness](09-Considering_Performance.md)
+* [Considering Correctness](09-Considering_Correctness.md)
 * [Enable Scripting](10-Enable_Scripting.md)
 * [Further Reading](11-Further_Reading.md)
 * [Final Thoughts](12-Final_Thoughts.md)