num: Fix the documentation of abs_sub.

Use proper argument names and unbackquote the word "zero" because it is
not an identifier.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #16233 : omasanori/rust/abs_sub, r=steveklabnik

Use proper argument names and unbackquote the word "zero" because it is not an identifier.