gitignore.txt: clarify default core.excludesfile path

The default core.excludesfile path is $XDG_CONFIG_HOME/git/ignore.
$HOME/.config/git/ignore is used if XDG_CONFIG_HOME is empty or unset,
as described later in the document.
Signed-off-by: NTodd Zullinger <tmz@pobox.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitignore.txt

@@ -7,7 +7,7 @@ gitignore - Specifies intentionally untracked files to ignore
 
 SYNOPSIS
 --------
-$HOME/.config/git/ignore, $GIT_DIR/info/exclude, .gitignore
+$XDG_CONFIG_HOME/git/ignore, $GIT_DIR/info/exclude, .gitignore
 
 DESCRIPTION
 -----------