libctf: fix a comment typo

ctf_update has been called ctf_serialize for years now.

libctf/

	* ctf-impl.h: Fix comment typo.

libctf/ctf-impl.h

diff --git a/libctf/ctf-impl.h b/libctf/ctf-impl.h
index dc57d6f..b712331 100644
--- a/libctf/ctf-impl.h
+++ b/libctf/ctf-impl.h
@@ -199,13 +199,13 @@
 } ctf_err_warning_t;
 
 /* Atoms associate strings with a list of the CTF items that reference that
-   string, so that ctf_update() can instantiate all the strings using the
+   string, so that ctf_serialize() can instantiate all the strings using the
    ctf_str_atoms and then reassociate them with the real string later.
 
    Strings can be interned into ctf_str_atom without having refs associated
    with them, for values that are returned to callers, etc.  Items are only
-   removed from this table on ctf_close(), but on every ctf_update(), all the
-   csa_refs in all entries are purged.  */
+   removed from this table on ctf_close(), but on every ctf_serialize(), all
+   the csa_refs in all entries are purged.  */
 
 typedef struct ctf_str_atom
 {