Update Doxygen Project Version
Update Doxygen Project Version

 * Change Project Version to 20.08

Signed-off-by: Ryan OShea <78edf9889e9e5db3b98905d7b2666c47635de0f0@arm.com>
Change-Id: I02ceca1995bdefb98d73511748fb2142cd27969e

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn