Add seed to custom_botorch_model_in_ax tutorial and clean up copy (#2131)

Summary:
Pull Request resolved: #2131

* Add seed so it won't fail (addressing T170755883)
* Clean up the text
* Only illustrate usage of MBM; don't show an alternative

Reviewed By: saitcakmak

Differential Revision: D51531072

fbshipit-source-id: 2fac6bbc8db95ea66fe02527a651c46c19ee44e7

Author: esantorella