Trailing register/define key mis-documented

[dbranford]

The manual gives acro/trailing/define as the key to add additional trailing characters but this is undefined and the source uses acro/trailing/register instead

https://github.com/cgnieder/acro/blob/9a56e414b0eb82ff89241decc66da9620f2292df/doc/acro-manual.tex#L1587 https://github.com/cgnieder/acro/blob/d2479bb10855d57984aabbd75762db79bb058c9a/code/acro.tools.code.tex#L486