Fix relative symlink paths when creating links

#335 introduced a bug when creating a backlink in notes files pointed to a symbolic link.

(org-brain-entry-path choice) already uses file-truename and thus returns the actual source path. buffer-file-name returns the path in the symbolic link, creating the inconsistent match.

For both paths to match, this change simply adds a file-truename call to get the actual source of buffer-file-name.