Missing line in 'Open-loop optimal control' docs

[flo-maier]

Describe the bug
On readthedocs there is a line missing in the code which leads to 'ValueError: not enough values to unpack' (expected 2, got 0) (https://c3-toolset.readthedocs.io/en/latest/optimal_control.html)

To Reproduce
Steps to reproduce the behavior:

1. Execute the code from Setup of a two-qubit chip with C3 and Open-loop optimal control on readthedocs
2. See error

Expected behavior
No error

Screenshots

Desktop (please complete the following information):

• OS: Windows 10
• PyCharm 2020.3.4
• Anaconda 4.9.2 / Python 3.8.5

Solution
Add 'exp.pmap.set_opt_map(gateset_opt_map)' before 'opt.optimize_controls()'

[lazyoracle]

Closing because docs are consistent with current dev