ox: Escape a docstring apostrophe

* lisp/ox.el (org-export-with-smart-quotes): Escape an apostrophe to
prevent it from being displayed as a curved quote in the help buffer.

lisp/ox.el

@@ -632,7 +632,7 @@ This option can also be set with the SELECT_TAGS keyword."
 (defcustom org-export-with-smart-quotes nil
   "Non-nil means activate smart quotes during export.
 This option can also be set with the OPTIONS keyword,
-e.g., \"':t\".
+e.g., \"\\=':t\".
 
 When setting this to non-nil, you need to take care of
 using the correct Babel package when exporting to LaTeX.