Documentation: avoid xmlto input error

Do not write literal "~/" or "~user" but use "{tilde}/" and "{tilde}user";
otherwise the text between them gets enclosed in
"<subscript>...</subscript>".

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

Documentation/config.txt

@@ -380,8 +380,8 @@ Common unit suffixes of 'k', 'm', or 'g' are supported.
 core.excludesfile::
 	In addition to '.gitignore' (per-directory) and
 	'.git/info/exclude', git looks into this file for patterns
-	of files which are not meant to be tracked.  "~/" is expanded
-	to the value of `$HOME` and "~user/" to the specified user's
+	of files which are not meant to be tracked.  "{tilde}/" is expanded
+	to the value of `$HOME` and "{tilde}user/" to the specified user's
 	home directory.  See linkgit:gitignore[5].
 
 core.editor::
@@ -667,7 +667,7 @@ color.ui::
 
 commit.template::
 	Specify a file to use as the template for new commit messages.
-	"~/" is expanded to the value of `$HOME` and "~user/" to the
+	"{tilde}/" is expanded to the value of `$HOME` and "{tilde}user/" to the
 	specified user's home directory.
 
 diff.autorefreshindex::