inet: fix typo

Note: btw submitted a number of other things in this area that haven't
made it into the tree, so I'm making an exception to the no typo rule
since it was done in that context.

Submitted by: btw (Tiwei Bie GSOC 2015 so unsure what to use for author)
Differential Revision: https://reviews.freebsd.org/D3510