Fix math display in ExampleEquilibrationLogic

Fix `'_' allowed only in math mode` in https://amici.readthedocs.io/en/latest/examples/example_steady_states/ExampleEquilibrationLogic.html

Was still okay in https://amici.readthedocs.io/en/v0.34.2/examples/example_steady_states/ExampleEquilibrationLogic.html