bug#17495: chgrp: mention of being a member of the target group

[Jim Meyering]

Looks fine. Two nits barely worth mentioning: one in the .texi file:
  s/group in/group of/

one in the log:
  s/\*man/* man/

Also, in the relative formality of documentation,
it's slightly better to write "It is" than "It's"