Fix Issue #2410 - Unlinked References doesnt work if the org-roam-directory has a space in it.

[vikram-mandyam]

Fix Issue #2410
In the function org-roam-unlinked-references-section the org-roam-directory variable is concated to form the rg_command. This doesn't cover any spaces in the org-roam-directory path.
So if the path is /Users/user123/MyDrive/My Files/org-mode/org-roam, due to the space in My Files rg returns an error:

rg: /Users/user123/MyDrive/My: No such file or directory (os error 2)
rg: Files/org-mode/org-roam: No such file or directory (os error 2)

This is due to the lines here:

(rg-command (concat "rg -L -o --vimgrep -P -i "
                               (mapconcat (lambda (glob) (concat "-g " glob))
                                          (org-roam--list-files-search-globs org-roam-file-extensions)
                                          " ")
                               (format " '\\[([^[]]++|(?R))*\\]%s' "
                                       (mapconcat (lambda (title)
                                                    (format "|(\\b%s\\b)" (shell-quote-argument title)))
                                                  titles ""))
                               org-roam-directory))