Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/01.plane-refracting-surface/02.plane-refracting-surface-overview/cheatsheet.en.md

@@ -215,27 +215,27 @@ in Figures 3 and 4, and 5 and 6 below, and review NOTE 1.
 [](thin-spherical-surface-1.png)<br>
 Fig. 4.
 
-[](thin-spherical-surface-2)<br>
+[](thin-spherical-surface-2.png)<br>
 Fig. 5.
 
-[](thin-spherical-surface-3)<br>
+[](thin-spherical-surface-3.png)<br>
 Fig. 6.
 
-[](thin-spherical-surface-4)<br>
+[](thin-spherical-surface-4.png)<br>
 Fig. 7.
 
 * with virtual objects
 
-[](thin-spherical-surface-5)<br>
+[](thin-spherical-surface-5.png)<br>
 Fig. 8.
 
-[](thin-spherical-surface-6)<br>
+[](thin-spherical-surface-6.png)<br>
 Fig. 9.
 
-[](thin-spherical-surface-7)<br>
+[](thin-spherical-surface-7.png)<br>
 Fig. 10.
 
-[](thin-spherical-surface-8)<br>
+[](thin-spherical-surface-8.png)<br>
 Fig. 11.