oc-csl: Allow nbsp after locator label

* lisp/oc-csl.el (org-cite-csl--label-regexp): Allow nbsp between
locator label and value.

Reported-by: M. ‘quintus’ Gülker <post+orgmodeml@guelker.eu>
<http://lists.gnu.org/r/emacs-orgmode/2021-10/msg00423.html>

lisp/oc-csl.el

@@ -270,11 +270,12 @@ If nil then the Chicago author-date style is used as a fallback.")
 (defconst org-cite-csl--label-regexp
   ;; Prior to Emacs-27.1 argument of `regexp' form must be a string literal.
   ;; It is the reason why `rx' is avoided here.
-  (rx-to-string `(seq word-start
-                      (regexp ,(regexp-opt (mapcar #'car org-cite-csl--label-alist) t))
-                      (0+ digit)
-                      (or word-start line-end (any ?\s ?\t)))
-                t)
+  (rx-to-string
+   `(seq word-start
+         (regexp ,(regexp-opt (mapcar #'car org-cite-csl--label-alist) t))
+         (0+ digit)
+         (or word-start line-end space ? ))
+   t)
   "Regexp matching a label in a citation reference suffix.
 Label is in match group 1.")