Add missing comma in docstring

lisp/transient.el

@@ -2513,7 +2513,7 @@ value.  Otherwise return CHILDREN as is."
                   ;; We cannot use `current-prefix-arg' because it is set
                   ;; too late (in `command-execute'), and if it were set
                   ;; earlier, then we likely still would not be able to
-                  ;; rely on it and `prefix-command-preserve-state-hook'
+                  ;; rely on it, and `prefix-command-preserve-state-hook'
                   ;; would have to be used to record that a universal
                   ;; argument is in effect.
                   (not prefix-arg)))