[docs] Code overflows the parent container

[shamin]

In documentation page https://docs.graphene-python.org/en/latest/types/enums/
code overflows the parent container and causes horizontal scrolling.

Caused by the long code in Usage with Python Enums section
https://docs.graphene-python.org/en/latest/types/enums/#usage-with-python-enums

[ekampf]

Fixed