covers: SPI: fix front cover color (inconsistent with doc file)

e1795dca · Pierre-Louis Roman · d07c4fa1 · e1795dca · d07c4fa1 · e1795dca
Commit e1795dca authored Oct 07, 2020 by Pierre-Louis Roman
--- a/Couverture-these/ecoles-doctorales/SPI/image-fond-garde-initial.png
+++ b/Couverture-these/ecoles-doctorales/SPI/image-fond-garde-initial.png
--- a/Couverture-these/ecoles-doctorales/SPI/image-fond-garde.png
+++ b/Couverture-these/ecoles-doctorales/SPI/image-fond-garde.png