incorrect model path in relaxFBA

[marouenbg]

Model path is local in relaxFBA tutorial. If the model is available in the CBT, I would change the path.

I hereby confirm that I have:

• [ x] Tried to solve the issue on my own
• [ x] Retried to run my code with the latest version of The COBRA Toolbox
• [x ] Checked that a similar issue has not already been opened

[laurentheirendt]

Discussed internally. Thanks @marouenbg.