Update Appendix-complex_numbers.md

Appendix-complex_numbers.md

@@ -25,10 +25,10 @@ $$ (eq3)
 
 In fact a complex number has two coordinates and can be graphically displayed as a point in a plane, the complex plane. This rectangular or the Cartesian coordinates are plotted along the real and the imaginary axis (see {numref}`fig:D1`). 
 
-```{figure} /Fig-app/figD1.png
+```{figure} /Fig-app/FigD1.png
 ---
 width: 400px
-name: fig:D1
+name: fig:FigD1
 ---
 Complex plane
 ```