nlist.3: Replace a.out(5) Xref with elf(5)

ELF support was added to nlist() in 1997, and a.out support was removed
in 2020.  The man page was not updated for either of these changes.

Fixes: 77909f5978 ("Initial elf nlist support, ...")
Fixes: 86cfa7e70b ("nlist: retire long-obsolete aout support")
Sponsored by: The FreeBSD Foundation

lib/libc/gen/nlist.3

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 19, 1994
+.Dd May 18, 2026
 .Dt NLIST 3
 .Os
 .Sh NAME
@@ -43,7 +43,7 @@ The
 function
 retrieves name list entries from the symbol table of an
 executable file (see
-.Xr a.out 5 ) .
+.Xr elf 5 ) .
 The argument
 .Fa \&nl
 is set to reference the