doc: Rename architectural diagram

It should not be specific to old gluster version anyway and the dot in name
causes trouble to pandoc/LaTeX:

pandoc: Error producing PDF from TeX source.
! LaTeX Error: Unknown graphics extension: .2_Architecture.png.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...

l.98 ...ages/640px-GlusterFS_3.2_Architecture.png}

Change-Id: I9d222a1628e1c14b004c059733fabd7b70c86f26
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5476
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>