libctf: fix a comment typo

libctf: fix a comment typo

ctf_dedup's intern() function does not return a dynamically allocated
string, so I just spent ten minutes auditing for obvious memory leaks
that couldn't actually happen.  Update the comment to note what it
actually returns (a pointer into an atoms table: i.e. possibly not
a new string, and not so easily leakable).

libctf/
	* ctf-dedup.c (intern): Update comment.