dir.c: fix typos in core.excludesfile comment

Make it easier to find references to core.excludesfile and the default
$XDG_CONFIG_HOME/git/ignore path.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -2475,7 +2475,7 @@ void setup_standard_excludes(struct dir_struct *dir)
 {
 	dir->exclude_per_dir = ".gitignore";
 
-	/* core.excludefile defaulting to $XDG_HOME/git/ignore */
+	/* core.excludesfile defaulting to $XDG_CONFIG_HOME/git/ignore */
 	if (!excludes_file)
 		excludes_file = xdg_config_home("ignore");
 	if (excludes_file && !access_or_warn(excludes_file, R_OK, 0))