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.

1 files changed, 1 insertions, 1 deletions

diff --git a/lisp/textmodes/flyspell.el b/lisp/textmodes/flyspell.el
index e5a7639e204..37f2245eded 100644
--- a/lisp/textmodes/flyspell.el
+++ b/lisp/textmodes/flyspell.el
@@ -72,7 +72,7 @@ Detection of repeated words is not implemented in
   "Non-nil means Flyspell matches duplicate words case-insensitively."
   :group 'flyspell
   :type 'boolean
-  :version 27.1)
+  :version "27.1")
 
 (defcustom flyspell-mark-duplications-exceptions
   '((nil . ("that" "had")) ; Common defaults for English.