Fixing the images in the PCMM section of the user guide.

I'm not sure if pcmm-technical-arch-overview-helium.png is the intended
image, but if not it's easy to replace later. The other image was
referred to as a jpg was was a png.

Change-Id: I8af6716f6ea42edada53fb0b6a1aa85008e823b1
Signed-off-by: Colin Dixon <colin@colindixon.com>