doc: replace @hashchar{} with actual hash character

doc: replace @hashchar{} with actual hash character

Very old makeinfo-4.13 fails with:
  ./doc/coreutils.texi:2286: Unknown command `hashchar'.
  ./doc/coreutils.texi:2286: Misplaced {.
  ./doc/coreutils.texi:2286: Misplaced }.

Reported Bernhard Voelker in
https://lists.gnu.org/r/coreutils/2019-03/msg00016.html .

* doc/coreutils.texi (basenc invocation): Replace @hashchar{} with
actual hash character.  The special syntax is only required
when referring to #line directives.