[Python] Add example of how to set a schema field to a specific value

[thisisnic]

e.g.

fields = [
    pa.field('foo', pa.int32()),
    pa.field('bar', pa.string()),
    pa.field('baz', pa.list_(pa.int8()))
]
s1 = pa.schema(fields)

s2 = s1.set(0, s1.field(0).with_type(pa.int64()))

See apache/arrow#42012 (comment) for more informaiton