Unit code for pieces should be "H87" instead of "C62"

[AntonHermann]

Hey there, first of all, thanks for your work on this project :)

I think the unit code for "pieces" should be "H87" instead of "C62", which is the unit code for "one".

Here is an excerpt from the code lists spreadsheet:

Code	Name
C62	one
H87	piece

I don't know wheter there is much practical difference between these two, but the comment in the README is slightly wrong imo: https://github.com/pretix/python-drafthorse/blob/a62b2e86a84cbc78c2211fcaff975cd5e319394f/README.rst?plain=1#L102-L105

LG, Anton