Update box-plots.md

[patelvyom]

• There is no need for passing dummy variable y when stats have already been computed
• upperfence values don't match the resulting figure.
• It could be simplified even further by getting rid of the extra call to add_trace but I've kept it to follow the documentation styling.
• It might be worth it to separate the example in two cases: (i) with only q1, median, q3 provided, (ii) with all values provided.