update figure in access internal page

[schoffelen]

The figure with the screenshot of a putty session needs to be updated. Currently it still shows mentat001 in the figure, should be mentat006 for consistency.

https://github.com/Donders-Institute/hpc-wiki-v2/blob/master/docs/cluster_howto/access-internal.rst

I can do it, but I'd rather supervise one of the TG people who have never done this before, so that they can work on their skill set.