man: Replace references to a.out(5) with elf(5)

The a.out(5) support in nlist(3) got removed some time ago, and
there is now only elf(5) support.

Signed-off-by: Guillem Jover <guillem@hadrons.org>

TODO

@@ -6,7 +6,6 @@
   - timeconv?
 * Add a README.import file.
 * Update man pages:
-  - Fix references to a.out(5) and inline needed struct definitions.
   - Document when each interface was added on every BSD, and libbsd.
 * Handle LFS properly. By default the library emits LFS objects, but might
   be used by non-LFS objects. We should either provide foo and foo64

man/nlist.3bsd

@@ -51,7 +51,7 @@ 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
@@ -74,7 +74,7 @@ if the file
 .Fa filename
 does not exist or is not executable, the returned value is \-1.
 .Sh SEE ALSO
-.Xr a.out 5
+.Xr elf 5
 .Sh HISTORY
 A
 .Fn nlist