Well, I found it finally. The file name is
tpl_header.php which is inside
tpl folder.
Here's what I changed, just in case someone else may have the same concern.
tpl_link(
DOKU_BASE,
'<img src="'.$logo.'" '.$logoSize[3].' alt="" /> <span>'.$conf['title'].'</span>',
'accesskey="h" title="[H]"'
);