lisp/hexl.el: Fix typos in docstrings.

lisp/hexl.el: Fix typos in docstrings.

(hexl-address-region, hexl-ascii-region)
(hexl-goto-hex-address, hexl-backward-char, hexl-forward-char)
(hexl-backward-short, hexl-forward-short, hexl-backward-word)
(hexl-forward-word, hexl-previous-line, hexl-next-line):
Use "Hexl mode" for non-hyperlinked hexl-mode references in docstrings.
(hexl-mode): Doc fix.
(hexl-ascii-start-column, hexl-beginning-of-line, hexl-end-of-line)
(hexl-mode-ruler): Fix typos in docstrings.