fix: Subplot in Chapter 1 gives Matplotlib warning

[kevinalh]

Fix a Matplotlib 3.3 deprecation warning in the beginning of the Chapter 1 notebooks.

The exact warning is:

MatplotlibDeprecationWarning: Passing non-integers as three-element position specification is deprecated since 3.3 and will be removed two minor releases later.

The simple fix is done by using floor division with // so that the type is int and not float. Since it's the first chapter it's particularly important to keep it clean.

[HVoltBb]

although floor division works here, since we have an even number of elements in n_trials. I think the correct code should be a ceiling division, which works for both even and odd numbers.
Try the double inverted ceiling division: -(-len(n_trials)//2)