Update location of SSI logo in people.html

[slel]

This is to fix the SSI logo not displaying at https://cadabra.science/people.html

The SSI logo seems to have moved (into a new images subdirectory):

• old: https://www.software.ac.uk/themes/ssi/ssi_logo_with_name-small2.png
• new: https://www.software.ac.uk/themes/ssi/images/ssi_logo_with_name-small2.png

[kpeeters]

I have decided to include the logo into the cadabra tree to avoid this in the future. Thanks for drawing my attention to this.

[slel]

Fixed by commit 3929781.