Fix build error: use string :version in defcustom

Fix build error: use string :version in defcustom

* lisp/textmodes/flyspell.el (flyspell-case-fold-duplications): Use a
  string value for the :version keyword to `defcustom'.  Otherwise,
  building Emacs will fail with an error like this:

    Scanning ./textmodes for custom
    Scanning ./url for custom
    Scanning ./vc for custom
    Generating ./cus-load.el...
    Version must be a string
    make[2]: *** [Makefile:152: cus-load.el] Error 255

This follows up to Reuben Thomas's commit 61f3a4b4fc of 10 Sep 2018.