Improving docstring for defface flyspell-duplicate

[Andrew M. Scott]

The docstring for defface flyspell-duplicate in flyspell.el says:

  "Face used for marking a misspelled word that appears twice in the
  buffer.

Replacing "twice" above with "two or more times" would better reflect
the current behavior of the code 

e.g. when I M-x flyspell-buffer, I regularly see more than two of the same
words colored with defface flyspell-duplicate.

Cheers,
Andy Scott