org-agenda: Fix `org-agenda-filter-by-category'

* lisp/org-agenda.el (org-agenda-filter-by-category): Use
`org-agenda-get-category' to get the category at current line.

Link: https://orgmode.org/list/CAOQTW-MHsnEak_Oj_iyKBsnm6_j4WzOgThM_58a__uu10KenbQ@mail.gmail.com

lisp/org-agenda.el

@@ -8211,7 +8211,7 @@ filter."
   (if (and org-agenda-filtered-by-category
 	   org-agenda-category-filter)
       (org-agenda-filter-show-all-cat)
-    (let ((cat (org-no-properties (org-get-at-eol 'org-category 1))))
+    (let ((cat (org-no-properties (org-agenda-get-category))))
       (cond
        ((and cat strip)
         (org-agenda-filter-apply