Update release version in Doxygen

Change-Id: I185aab58e193b5d02ebb424724f9255003aa600a
Signed-off-by: Sang-Hoon Park <sang-hoon.park@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/3770
Reviewed-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
Reviewed-by: Pablo Marquez <pablo.tello@arm.com>
Tested-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
diff --git a/docs/Doxyfile b/docs/Doxyfile
index 7f5aa5b..ef8966c 100644
--- a/docs/Doxyfile
+++ b/docs/Doxyfile
@@ -38,7 +38,7 @@
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 0.0
+PROJECT_NUMBER         = 20.08
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a